Abstract
Drosophila melanogaster sarcomere length short (SALS) is a recently identified Wiskott-Aldrich syndrome protein homology 2 (WH2) domain protein involved in skeletal muscle thin filament regulation. SALS was shown to be important for the establishment of the proper length and organization of sarcomeric actin filaments. Here, we present the first detailed characterization of the biochemical activities of the tandem WH2 domains of SALS (SALS-WH2). Our results revealed that SALS-WH2 binds both monomeric and filamentous actin and shifts the monomer-filament equilibrium toward the monomeric actin. In addition, SALS-WH2 can bind to but fails to depolymerize phalloidin- or jasplakinolide-bound actin filaments. These interactions endow SALS-WH2 with the following two major activities in the regulation of actin dynamics: SALS-WH2 sequesters actin monomers into non-polymerizable complexes and enhances actin filament disassembly by severing, which is modulated by tropomyosin. We also show that profilin does not influence the activities of the WH2 domains of SALS in actin dynamics. In conclusion, the tandem WH2 domains of SALS are multifunctional regulators of actin dynamics. Our findings suggest that the activities of the WH2 domains do not reconstitute the presumed biological function of the full-length protein. Consequently, the interactions of the WH2 domains of SALS with actin must be tuned in the cellular context by other modules of the protein and/or sarcomeric components for its proper functioning.
Highlights
The basic myofibrillar contractile units, sarcomeres, orchestrate the function of striated muscle
Wiskott-Aldrich syndrome protein homology 2 (WH2) Domains of Sarcomere Length Short (SALS) Bind Actin Monomers—To begin the biochemical characterization of the tandem WH2 domains of SALS (SALS-WH2), we addressed the question whether, similar to other WH2 domains, SALS-WH2 is able to interact with actin monomers
We investigated the biochemical activities of the WH2 domains and the proline-rich motif of D. melanogaster SALS required for sarcomeric actin assembly
Summary
The basic myofibrillar contractile units, sarcomeres, orchestrate the function of striated muscle. In skeletal muscles two WH2 domain proteins have been implicated in the regulation of assembly and organization of sarcomeric actin filaments as follows: the vertebrate specific leiomodin (Lmod) and the Drosophila melanogaster Sarcomere Length Short (SALS) proteins (18 –20). Each Lmod has a single WH2 domain; they contain additional actin-binding regions, as well as Tmodbinding domains Both Lmod and Lmod were shown to play an important role in the organization of sarcomeric actin filaments (19 –21). LSALS possesses two tandem WH2 domains and an upstream prolinerich region (Fig. 1B), and the sSALS covers the C-terminal half of the long isoform containing only a single WH2 domain and lacking the proline-rich element. The activities of the WH2 domains of SALS were not analyzed in detail
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