Androcam replaces calmodulin as a tissue-specific myosin VI light
chain on the actin cones that mediate D. melanogaster spermatid
individualization. We show that the androcam structure and its
binding to the myosin VI structural (Insert 2) and regulatory (IQ)
light chain sites are distinct from those of calmodulin and provide
a basis for specialized myosin VI function. The androcam N lobe
noncanonically binds a single Ca2þ and is locked in a “closed” conformation,
causing androcam to contact the Insert 2 site with its C
lobe only. Androcam replacing calmodulin at Insert 2 will increase
myosin VI lever arm flexibility, which may favor the compact monomeric
form of myosin VI that functions on the actin cones by facilitating
the collapse of the C-terminal region onto the motor
domain. The tethered androcam N lobe could stabilize the monomer
through contacts with C-terminal portions of the motor or
recruit other components to the actin cones. Androcam binds the
IQ site at all calcium levels, constitutively mimicking a conformation
adopted by calmodulin only at intermediate calcium levels. Thus,
androcam replacing calmodulin at IQ will abolish a Ca2þ-regulated,
calmodulin-mediated myosin VI structural change. We propose that
the N lobe prevents androcam from interfering with other calmodulin-
mediated Ca2þ signaling events. We discuss how gene duplication
and mutations that selectively stabilize one of the many conformations
available to calmodulin support the molecular evolution
of structurally and functionally distinct calmodulin-like proteins