Bmc Cell Biology Loss of Dictyostelium Hspc300 Causes a Scar-like Phenotype and Loss of Scar Protein

Background: SCAR/WAVE proteins couple signalling to actin polymerization, and are thus fundamental to the formation of pseudopods and lamellipods. They are controlled as part of a fivemembered complex that includes the tiny HSPC300 protein. It is not known why SCAR/WAVE is found in such a large assembly, but in Dictyostelium the four larger subunits have different, clearly delineated functions. Results: We have generated Dictyostelium mutants in which the HSPC300 gene is disrupted. As has been seen in other regulatory complex mutants, SCAR is lost in these cells, apparently by a posttranslational mechanism, though PIR121 levels do not change. HSPC300 knockouts resemble scar mutants in slow migration, roundness, and lack of large pseudopods. However hspc300-colonies on bacteria are larger and more similar to wild type, suggesting that some SCAR function can survive without HSPC300. We find no evidence for functions of HSPC300 outside the SCAR complex. Conclusion: HSPC300 is essential for most SCAR complex functions. The phenotype of HSPC300 knockouts is most similar to mutants in scar, not the other members of the SCAR complex, suggesting that HSPC300 acts most directly on SCAR itself. Background The WASP/SCAR family of proteins are key regulators of actin polymerisation, connecting signalling molecules to the activation of the Arp2/3 complex. SCAR/WAVE proteins, in particular, play an important role in the regulation of actin dynamics at the leading edges of moving cells. Biochemical studies in a range of organisms demonstrate that SCAR/WAVE is found in a 1:1:1:1:1 complex with four other proteins (PIR121, Nap1, Abi2 and HSPC300) [1,2]. It is becoming clear, in particular from studies in Dictyostelium, that individual components of the complex regulate SCAR through different signalling pathways and that some may also have additional SCAR independent functions in vivo. [3-6] Most evidence now suggests that all members of the complex are needed for the correct localisation and function of SCAR [7,8]. The smallest SCAR complex member, HSPC300, ranges from 68 to 110 amino acids in length, giving a size of between 8 and 14 kDa. Surprisingly little is known about its contribution to SCAR complex function and stability, perhaps because of the experimental difficulty associated with its smallness. Several studies investigating the function of the plant HSPC300 homologue, BRICK1, have Published: 19 February 2009 BMC Cell Biology 2009, 10:13 doi:10.1186/1471-2121-10-13 Received: 9 July 2008 Accepted: 19 February 2009 This article is available from: http://www.biomedcentral.com/1471-2121/10/13 © 2009 Pollitt and Insall; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.