Conformational and metal-binding properties of androcam, a testis- specific, calmodulin-related protein from Drosophila

Stephen R. Martin, Alan Q. Lu, Jie Xiao, Jens Kleinjung, Kathy Beckingham, Peter M. Bayley

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


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 calcium-free 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 X 103 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 103-105 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.

Original languageEnglish (US)
Pages (from-to)2444-2454
Number of pages11
JournalProtein Science
Issue number11
StatePublished - 1999
Externally publishedYes


  • Calcium binding
  • Calmodulin
  • Modeling
  • Spectroscopy, stability, testis

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology


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