The amino-terminal fragment of gelsolin is cross-linked to Cys-374 of actin in the EGTA-resistant actin-gelsolin complex

Actin's C-terminus coordinates actin structural changes and functions

Actin is essential to eukaryotic cellular processes. Actin's C-terminus appears to play a direct role in modulating actin's structure and properties, facilitating the binding and function of actin-binding proteins (ABPs). The structural and functional characterization of filamentous actin's C-terminus has been impeded by its inherent flexibility, as well as actin's resistance to crystallization for x-ray diffraction and the historical resolution constraints associated with electron microscopy. Many biochemical studies have established that actin's C-terminus must retain its flexibility and structural integrity to modulate actin's structure and functions. For example, C-terminal structural changes are known to affect nucleotide binding and exchange, as well as propagate actin structural changes throughout extensive allosteric networks, facilitating the binding and function of ABPs. Advances in electron microscopy have resulted in high-resolution structures of filamentous actin, providing insights into subtle structural changes that are mediated by actin's C-terminus. Here, we review existing knowledge establishing the importance of actin's C-terminus within actin structural changes and functions and discuss how modern structural characterization techniques provide the tools to understand the role of actin's C-terminus in cellular processes.

Conformational and functional studies of three gelsolin subdomain-1 synthetic peptides and their implication in actin polymerization

Gelsolin, a calcium and inositol phospholipid-sensitive protein, regulates actin filament length. Its activity is complex (capping, severing, etc.) and is supported by several functional domains. The N-terminal domain alone (S1), in particular, is able to impede actin polymerization. Our investigations were attempted to precise this inhibitory process by using synthetic peptides as models mimicking gelsolin S1 activity. Three peptides issued from S1 and located in gelsolin-actin interfaces were synthesized. The peptides (15-28, 42-55, and 96-114 sequences) were tested for their conformational and actin binding properties. Although the three peptides interact well with actin, only peptide 42-55 affects actin polymerization. A detailed kinetic study shows that the latter peptide essentially inhibits the nucleation step during actin polymerization. In conclusion, the present work shows that the binding of a synthetic peptide to a small sequence located outside the actin-actin interface is essential in the actin polymerization process.

Definition of the EGTA-independent interface involved in the serum gelsolin-actin complex

The gelsolin-actin complex in the presence of Ca2+ revealed at least three interacting sites on the gelsolin molecule located in the S1, S2-3, and S4-6 domains. In the presence of EGTA, the N-terminal domain of gelsolin is known to be involved. However, the corresponding site on the surface of actin is poorly defined. The present result locates the Ca(2+)-independent plasma gelsolin-binding site on the actin surface. Natural and synthetic actin peptides were tested for their possible interaction with gelsolin and monitored by fluorescence anisotropy measurements and e.l.i.s.a. The interface was thus located within the 360-372 actin sequence near the C-terminal extremity. In addition, we used a chymotryptic digest of gelsolin and determined that its N-terminal domain (S1) was implicated in this interface. We conclude that the interaction of the 41-126 region of plasma gelsolin is the counterpart of the 360-372 sequence in subdomain 1 of actin.