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

Protein Sci. 1999 Nov;8(11):2444-54. doi: 10.1110/ps.8.11.2444.


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 [Ka(Mg) approximately 3 x 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 alpha-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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Binding Sites
  • Calcium / metabolism
  • Calcium-Binding Proteins / chemistry*
  • Calcium-Binding Proteins / metabolism*
  • Calmodulin / chemistry*
  • Circular Dichroism
  • Drosophila Proteins*
  • Drosophila melanogaster
  • Kinetics
  • Male
  • Molecular Sequence Data
  • Peptide Fragments / chemistry
  • Protein Conformation
  • Sequence Alignment
  • Sequence Homology, Amino Acid
  • Testis


  • Acam protein, Drosophila
  • Calcium-Binding Proteins
  • Calmodulin
  • Drosophila Proteins
  • Peptide Fragments
  • Calcium