Hydrogen/Deuterium Exchange Mass Spectrometry Provides Insights into the Role of Drosophila Testis-Specific Myosin VI Light Chain AndroCaM

In Drosophila testis, myosin VI plays a special role, distinct from its motor function, by anchoring components to the unusual actin-based structures (cones) that are required for spermatid individualization. For this, the two calmodulin (CaM) light-chain molecules of myosin VI are replaced by androcam (ACaM), a related protein with 67% identity to CaM. Although ACaM has a similar bi-lobed structure to CaM, with two EF hand-type Ca²⁺ binding sites per lobe, only one functional Ca²⁺ binding site operates in the amino-terminus. To understand this light chain substitution, we used hydrogen-deuterium exchange mass spectrometry (HDX-MS) to examine dynamic changes in ACaM and CaM upon Ca²⁺ binding and interaction with the two CaM binding motifs of myosin VI (insert2 and IQ motif). HDX-MS reveals that binding of Ca²⁺ to ACaM destabilizes its N-lobe but stabilizes the entire C-lobe, whereas for CaM, Ca²⁺ binding induces a pattern of alternating stabilization/destabilization throughout. The conformation of this stable holo-C-lobe of ACaM seems to be a "prefigured" version of the conformation adopted by the holo-C-lobe of CaM for binding to insert2 and the IQ motif of myosin VI. Strikingly, the interaction of holo-ACaM with either peptide converts the holo-N-lobe to its Ca²⁺-free, more stable, form. Thus, ACaM in vivo should bind the myosin VI light chain sites in an apo-N-lobe/holo-C-lobe state that cannot fulfill the Ca²⁺-related functions of holo-CaM required for myosin VI motor assembly and activity. These findings indicate that inhibition of myosin VI motor activity is a precondition for transition to an anchoring function.