Assignment of the 1H, 13C and 15N resonances of Mlc1p from Saccharomices cerevisiae

N-lobe dynamics of myosin light chain dictates its mode of interaction with myosin V IQ1

Myosin V motors regulate secretion and cell division in eukaryotes. How MyoV activity is differentially regulated by essential and calmodulin light chain binding remains unclear. We have used NMR spectroscopy to compare the dynamic behavior of Mlc1p, a budding yeast essential light chain, with that of the Xenopus laevis calmodulin. Both proteins have a similar structure and bind similar target proteins but differ in the mechanism by which they interact with the myosin V IQ1. This interaction is essential for MyoV activity. Here, we show that the rigid conformation of the loop connecting the two EF-hand motifs of the Mlc1p N-lobe explains its differential ability to interact with myosin V IQ1. Moreover, we show that the maintenance of the N-lobe structure is required for the essential function of Mlc1p in vivo. These data show that the core characteristics of myosin light chain N-lobes differentiate Mlc1p and calmodulin binding capability.

Structural basis for the interaction of the myosin light chain Mlc1p with the myosin V Myo2p IQ motifs

Calmodulin, regulatory, and essential myosin light chain are evolutionary conserved proteins that, by binding to IQ motifs of target proteins, regulate essential intracellular processes among which are efficiency of secretory vesicles release at synapsis, intracellular signaling, and regulation of cell division. The yeast Saccharomyces cerevisiae calmodulin Cmd1 and the essential myosin light chain Mlc1p share the ability to interact with the class V myosin Myo2p and Myo4 and the class II myosin Myo1p. These myosins are required for vesicle, organelle, and mRNA transport, spindle orientation, and cytokinesis. We have used the budding yeast model system to study how calmodulin and essential myosin light chain selectively regulate class V myosin function. NMR structural analysis of uncomplexed Mlc1p and interaction studies with the first three IQ motifs of Myo2p show that the structural similarities between Mlc1p and the other members of the EF-hand superfamily of calmodulin-like proteins are mainly restricted to the C-lobe of these proteins. The N-lobe of Mlc1p presents a significantly compact and stable structure that is maintained both in the free and complexed states. The Mlc1p N-lobe interacts with the IQ motif in a manner that is regulated both by the IQ motifs sequence as well as by light chain structural features. These characteristic allows a distinctive interaction of Mlc1p with the first IQ motif of Myo2p when compared with calmodulin. This finding gives us a novel view of how calmodulin and essential light chain, through a differential binding to IQ1 of class V myosin motor, regulate this activity during vegetative growth and cytokinesis.

HACACO revisited: residual dipolar coupling measurements and resonance assignments in proteins

The revisited version of the HACACO experiment here presented, is more robust and straightforward to implement and continues to be, to a greater extent, a convenient tool for protein backbone resonance assignment. Additionally, it turns out to be a sensitive and accurate method to measure C(alpha)-H(alpha) residual dipolar couplings (RDCs). The performance of our new pulse scheme for measurement of RDCs was tested on two proteins with different secondary structures: one characterized by a high beta-sheet content, the second dominated by the presence of alpha-helices. In both examples the new method provided significantly more accurate data, compared to all previously published 3D techniques.