Hostname: page-component-7479d7b7d-qlrfm Total loading time: 0 Render date: 2024-07-13T23:39:09.149Z Has data issue: false hasContentIssue false

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
Get access

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

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)