Structure of androcam supports specialized interactions with myosin VI

Mobility capillary electrophoresis-native mass spectrometry reveals the dynamic conformational equilibrium of calmodulin and its complexes

Benefitting from the rapid evolution of artificial intelligence and structural biology, an expanding collection of high-resolution protein structures has greatly improved our understanding of protein functions. Yet, proteins are inherently flexible, and these static structures can only offer limited snapshots of their true dynamic nature. The conformational and functional changes of calmodulin (CaM) induced by Ca2+ binding have always been a focus of research. In this study, the conformational dynamics of CaM and its complexes were investigated using a mobility capillary electrophoresis (MCE) and native mass spectrometry (native MS) based method. By analyzing the ellipsoidal geometries of CaM in the solution phase at different Ca2+ concentrations, it is interesting to discover that CaM molecules, whether bound to Ca2+ or not, possess both closed and open conformations. Moreover, each individual CaM molecule actively "jumps" (equilibrium exchange) between these two distinct conformations on a timescale ranging from milli- to micro-seconds. The binding of Ca2+ ions did not affect the structural dynamics of CaM, while the binding of a peptide ligand would stabilize CaM, leading to the observation of a single, compact conformation of the resulting protein complex. A target recognition mechanism was also proposed based on these new findings, suggesting that CaM's interaction with targets may favor a conformational selection model. This enriches our understanding of the binding principles between CaM and its numerous targets.

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.

Molecular evolution of the multiple calmodulin-like cal genes in C. elegans and in nematodes

Calmodulin (CaM) is a major EF hand containing intracellular calcium receptor in animals and plants; however, eukaryotes also express a number of related CaM-like proteins. We have previously characterized an embryonic phenotype of the single Caenorhabditis elegans CaM gene cmd-1, reported no visible RNAi phenotype for the four related cal-1 to cal-4 genes and started tissue-specific expression analyses of these proteins. In the present study, we analyzed evolutionary aspects of the previously reported CAL-1 to CAL-4 proteins, along with the four new CAL-5 to CAL-8 sequences retrieved from the worm database. Phylogenetic analyses suggest that all C. elegans CAL proteins arose from a CaM ancestor through repeated gene duplications, fusions and sequence divergence. The same holds, also, for the variable N-terminal extensions of the CAL-1 to CAL-4 proteins, which have evolved from the CaM-like core domain. We found 97 CAL homologs in different nematode clades and also detected two CAL-7-related sequences outside the nematodes. Moreover, the C. elegans-specific cal-6 gene, representing the most CaM-related sequence found in nematodes so far, harbours many deletions, insertions and sequence substitutions and is predicted, therefore, to be non-functional. These analyses provide an insight into a complex and dynamic origin of the multiple CAL genes in C. elegans and in nematodes and represent also a basis for further functional studies of these CaM-related sequences in nematodes.

Myosin light chains: Teaching old dogs new tricks

The myosin holoenzyme is a multimeric protein complex consisting of heavy chains and light chains. Myosin light chains are calmodulin family members which are crucially involved in the mechanoenzymatic function of the myosin holoenzyme. This review examines the diversity of light chains within the myosin superfamily, discusses interactions between the light chain and the myosin heavy chain as well as regulatory and structural functions of the light chain as a subunit of the myosin holoenzyme. It covers aspects of the myosin light chain in the localization of the myosin holoenzyme, protein-protein interactions and light chain binding to non-myosin binding partners. Finally, this review challenges the dogma that myosin regulatory and essential light chain exclusively associate with conventional myosin heavy chains while unconventional myosin heavy chains usually associate with calmodulin.