Address for correspondence and reprints: Michael E. Baker, Department of Medicine, M-023, University of California .... WALTER M. FITCH, reviewing editor.
Letter to the Editor Conservation of Amino Acid Sequences in Albumin: Is Albumin an Essential Protein? Michael E. Baker Department of Medicine, University of California, San Diego
Albumin, the most abundant protein in vertebrate serum, is thought to be a nonessential protein because other serum proteins take over albumin’s actions to bind and transport hydrophobic ligands such as fatty acids, bilirubin, and steroids and to maintain osmotic pressure in blood in “analbuminemic” humans (Gitlin and Gitlin 1975; Bowman et al. 1976) and rats (Nagase et al. 1979). Additional support for this idea comes from a recent analysis (Minghetti et al. 1985) of the amino acid sequences of rodent and human albumin and alpha-fetoprotein (AFP), a paralogue of albumin (Alexander-Eiferman et al. 198 1; Jagodzinski et al. 198 1; Morinaga et al. 1983). This analysis shows that albumin and AFP are accumulating amino acid changes about twofold and threefold, respectively, faster than does hemoglobin. In fact, the rate of change in AFP is about half that of pseudogenes and approaches that of fibrinopeptides (Minghetti et al. 1985). When segments of a protein undergo relatively rapid changes in their amino acid sequences, they are thought to be nonessential to the biological functioning of the protein. Thus, if most of albumin is undergoing rapid change, and if the absence of albumin does not lead to an obvious disease state, then it would seem that albumin no longer has an essential biological function in the organism. Here, I show that both of these premises are invalid, and I propose that albumin has essential, albeit still unelucidated, biological function(s). First, it has not truly been demonstrated that humans or rats can survive without albumin, because “analbuminemic” humans and rats have - lo-25 pg albumin/ml. Even though this concentration is > 1,000 fold less than the normal albumin concentration, it still is high when compared with the concentration of growth factors or other essential proteins in serum. Second, as shown in table 1, exons 12- 14, the last three coded exons in domain III of human albumin, are 50% identical to human AFP. Figure 1 shows the alignment of the 58 residues of exons 13 and 14 in human albumin and AFP. To put these numbers in perspective, note that alignment of human hemoglobin alpha and beta chains, a paralogous system, reveals 45% identities (Feng and Doolittle 1987). These genes diverged -400-450 Mya (Goodman et al. 1988), a date similar to that for the divergence of albumin and AFP. Thus, a 133-residue part of the human albumin and AFP (exons 12- 14) is changing more slowly than the alpha and beta chains of human hemoglobin-and clearly much less than fibrinopeptides and pseudogenes. From table 1 it is clear that other subdomains of human albumin and AFP, such as subdomains I-C, II-A, II-C, and II-D, also are under constraints as far as changes in amino acid sequences are concerned. Table 1 reveals that some albumin and AFP subdomains (e.g., I-A, I-D, II-B, and III-A) differ by 65%-77%, which serves to further emphasize that there are indeed constraints on changes in the sequences of other parts of albumin and AFP, parts that I suggest are involved in essential biological actions of albumin and AFP. Address for correspondence and reprints: Michael E. Baker, Department of Medicine, M-023, University of California, San Diego, La Jolla, California 92093. Mol. Biol. Ed. 6(3):321-323. 1989. 0 1989 by The University of Chicago. All rights reserved. 0737-4038/89/0603-009$02.00
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Table 1 Percent Identical Residues in Corresponding Albumin and AFP
Subdomains
of Human
SUBDOMAIN DOMAIN I ............... II ............... III ...............
A
B
C
D
23 49 35
37 35 48
45 48 55
30 45 50
NOTE.-subdomain I-A corresponds to exon 3. The subdomains continue in order to subdomain III-D, which corresponds to exon 14. Exon 14, which has 50% identity between human albumin and human AFP, is thought to be part of exon D. Exons 12-14 of human albumin and human AFP contain 133 residues, which are 50% identical without insertion of gaps. Only subdomain I-B required a gap.
Similarity between domains of albumin and AFP has been noted elsewhere (Alexander-Eiferman et al. 198 1; Jagodzinski et al. 198 1; Morinaga et al. 1983; Minghetti et al. 1985), without the interpretation presented here, owing, I think to the acceptance of the notion that albumin is a nonessential protein. Our hypothesis explains Takahashi et al.‘s ( 1987) report that the degree of change in the coded sequences of several albumin variants is similar to that found in essential proteins, a result that they found puzzling because coded sequences for a nonessential protein would not be expected to be conserved. By essential, I mean that the lack of albumin or AFP in an organism results in its death, just as lack of an essential enzyme is lethal. There is little effort toward uncovering these essential biological function(s), owing to the acceptance of the notion that albumin is nonessential. There are, however, several properties of albumin and AFP that could be involved in biological actions at concentrations < 10 pg/ml (Baker 1988). Both albumin (Weisiger et al. 198 1; Melsert et al. 1988; Reed and Burrington 1988) and AFP (Uriel et al. 1987) bind to the cell surface. Intracellular albumin binds tropomyosin (Gerhard et al. 1985). It is interesting that an albumin/bilirubin complex stimulates growth of liver cells (Diaz-Gil et al. 1987). These biological properties and the evidence that both albumin and AFP have parts that are constrained to the same degree as essential proteins provides, I think, a sound basis for more vigorously investigating albumin and AFP for essential biological function(s).
Albumin
528
AFP
533
Albumin
ALVELVKHKPKATKEQLKAV
MDDFAAFVEKCCKADDKET
AFP
Albumin
C ;
F ;
A a
E ;
E ;
G &
K Q
K ;
L ;
V ;
A s
A K
S
Q
+
R
A ;
A ;
L ;
G ;
L ;
585 590
AFP FIG. I.-Comparison of exons 13 and 14 of human albumin and AFP. A colon (:) denotes identities; a period (.) denotes conservative replacements. Of 58 possible matches there are 3 1 (53%) identities and seven ( 12%) conservative
replacements.
Letters to the Editor
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Acknowledgments
I thank Darrell Fanestil for his continuing interest and valuable discussion and Andrew Ansel for encouragement and financial support. LITERATURE
CITED
ALEXANDER-EIFERMAN, F., P. R. YOUNG, R. W. SCOTT,and S. M. TILGHMAN. 198 1. Intragenic amplification and divergence in the mouse a-fetoprotein gene. Nature 294:7 13-7 18. BAKER, M. E. 1988. Evolution of alpha-fetoprotein: sequence comparisons among AFP species and with albumin species. Tumour Biol. 9: 12 1- 136. BOWMAN,H., M. HERMODSON,C. A. HAMMOND,and A. G. MOTULSKY.1976. Analbuminemia in an American Indian girl. Clin. Genet. 9:5 13-526. DIAZ-GIL, J. J., J. G. GAVILANES,G. SANCHEZ,R. GARCIA-CANERO,J. M. GARCIA-SEGURA, L. SANTAMARIA,C. TRILLA, and P. ESCARTIN. 1987. Identification of a liver growth factor as an albumin-bilirubin complex. Biochem. J. 243:443-448. FENG, D.-F., and R. F. DOOLITTLE. 1987. Aligning amino acid sequences: comparison of commonly used methods. J. Mol. Evol. 25:351-360. GERHARD, M. D., P. M. DIGIROLAMO, and S. E. HITCHCOCK-DEGREGORI. 1985. Isolation and characterization of a tropmyosin binding protein from blood platelets. J. Biol. Chem. 260:322 l-3227. GITLIN, D., and J. D. GITLIN. 1975. Genetic alterations in the plasma proteins of man. Pp. 321-374 in F. W. PUTNAM, ed. The plasma proteins. Academic Press, New York. GOODMAN, M., J. REDWAYDON, J. CZELUSNIAK, T. SUZUKI, T. GOTOH, L. MOENS, F. SHISHIKURA,D. WALZ, and S. VINOGRADOV.1988. An evolutionary tree for invertebrate globin sequences. J. Mol. Evol. 27:236-249. JAGODZINSKI,L. L., T. D. SARGENT,M. YANG, C. GLACIUN, and J. BONNER. 198 1. Sequence homology between RNAs encoding rat a-fetoprotein and rat serum albumin. Proc. Natl. Acad. Sci. USA 78:3521-3525. MELSERT, R., J. W. HCKK~ERBRUGGE, and F. F. G. ROMMERTS. 1988. The albumin fraction of rat testicular fluid stimulates steroid production by isolated Leydig cells. Mol. Cell. Endocrinol. 59:22 l-23 1. MINGHETTI, P. P., S. W. LAW, and A. DUGAICZYK. 1985. The rate of molecular evolution of a-fetoprotein approaches that of pseudogenes. Mol. Biol. Evol. 2:347-358. MORINAGA, T., M. SAKAI, T. G. WEGMANN, and T. TAMAOIU. 1983. Primary structures of human a-fetoprotein and its mRNA. Proc. Natl. Acad. Sci. USA 80:4604-4608. NAGASE,S. K., K. SHIMAMUNE,and S. SHUMIYA. 1979. Albumin-deficient rat mutant. Science 205:590-59 1. REED, R. G., and C. M. BURRINGTON. 1988. The albumin receptor effect may be due to a surface-induced conformational change in albumin. FASEB J. 2:A320. TAKAHASHI,N., Y. TAKAHASHI, T. ISOBE, F. W. PUTNAM, M. FUJITA, C. SATOH, and J. V. NEEL. 1987. Amino acid substitutions in inherited albumin variants from Amerindian and Japanese populations. Proc. Natl. Acad. Sci. USA 84:800 l-8005. URIEL, J., J. NAVAL, and J. LABBORDA.1987. a-Fetoprotein-mediated transfer of arachidonic acid into cultured cloned cells derived from a rat rhabdomyosarcoma. J. Biol. Chem. 262: 3579-3585. WEISIGER,R., J. GOLLAN,and R. OCKNER. 198 1. Receptor for albumin on the liver cell surface may mediate uptake of fatty acids and other albumin-bound substances. Science 211: 10481051. WALTER M. FITCH, reviewing editor
Received August 26, 1988; revision received January 3, 1989
