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Conformational and metal‐binding properties of androcam, a testis‐specific, calmodulin‐related protein from drosophila
Author(s) -
Martin Stephen R.,
Kleinjung Jens,
Bayley Peter M.,
Lu Alan Q.,
Xiao Jie,
Beckingham Kathy
Publication year - 1999
Publication title -
protein science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.353
H-Index - 175
eISSN - 1469-896X
pISSN - 0961-8368
DOI - 10.1110/ps.8.11.2444
Subject(s) - calmodulin , calcium , calcium binding protein , chemistry , conformational change , biophysics , binding domain , biochemistry , ef hand , trypsin , binding site , protein structure , biology , enzyme , organic chemistry
Androcam is a testis‐specific protein of Drosophila melanogaster , with 67% sequence identity to calmodulin and four potential EF‐hand calcium‐binding sites. Spectroscopic monitoring of the thermal unfolding of recombinant calciumfree androcam shows a biphasic process characteristic of a two‐domain protein, with the apo‐N‐domain less stable than the apo‐C‐domain. The two EF hands of the C‐domain of androcam bind calcium cooperatively with 40‐fold higher average affinity than the corresponding calmodulin sites. Magnesium competes with calcium binding [ K a (Mg) ˜ 3 × 10 3 M −1 ]. Weak calcium binding is also detected at one or more N‐domain sites. Compared to apo‐calmodulin, apo‐androcam has a smaller conformational response to calcium and a lower α‐helical content over a range of experimental conditions. Unlike calmodulin, a tryptic cleavage site in the N‐domain of apo‐androcam remains trypsin sensitive in the presence of calcium, suggesting an altered calcium‐dependent conformational change in this domain. The affinity of model target peptides for androcam is 10 3 ‐10 5 times lower than for calmodulin, and interaction of the N‐domain of androcam with these peptides is significantly reduced. Thus, androcam shows calcium‐induced conformational responses typical of a calcium sensor, but its properties indicate calcium sensitivity and target interactions significantly different from those of calmodulin. From the sequence differences and the altered calcium‐binding properties it is likely that androcam differs from calmodulin in the conformation of residues in the second calcium‐binding loop. Molecular modeling supports the deduction that there are significant conformational differences in the N‐domain of androcam compared to calmodulin, and that these could affect the surface, conferring a different specificity on androcam in target interactions related to testis‐specific calcium signaling functions.