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

Published online by Cambridge University Press:  01 November 1999

STEPHEN R. MARTIN
Affiliation:
Division of Physical Biochemistry, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, United Kingdom
ALAN Q. LU
Affiliation:
Department of Biochemistry and Cell Biology, MS 140, Rice University, 6100 Main St., Houston, Texas 77005-1892
JIE XIAO
Affiliation:
Department of Biochemistry and Cell Biology, MS 140, Rice University, 6100 Main St., Houston, Texas 77005-1892
JENS KLEINJUNG
Affiliation:
Division of Physical Biochemistry, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, United Kingdom
KATHY BECKINGHAM
Affiliation:
Department of Biochemistry and Cell Biology, MS 140, Rice University, 6100 Main St., Houston, Texas 77005-1892
PETER M. BAYLEY
Affiliation:
Division of Physical Biochemistry, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, United Kingdom
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Abstract

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) ∼3 × 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.

Type
Research Article
Copyright
© 1999 The Protein Society

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