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J. Biol. Chem., Vol. 283, Issue 22, 15037-15046, May 30, 2008

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Interactions between the Yeast SM22 Homologue Scp1 and Actin Demonstrate the Importance of Actin Bundling in Endocytosis^*

Dana M. Gheorghe¹, Soheil Aghamohammadzadeh², Iwona I. Smaczynska-de Rooij³, Ellen G. Allwood, Steve J. Winder, and Kathryn R. Ayscough⁴

From the Departments of Molecular Biology and Biotechnology, Biomedical Science, University of Sheffield, Sheffield, S10 2TN, United Kingdom

The yeast SM22 homologue Scp1 has previously been shown to act as an actin-bundling protein in vitro. In cells, Scp1 localizes to the cortical actin patches that form as part of the invagination process during endocytosis, and its function overlaps with that of the well characterized yeast fimbrin homologue Sac6p. In this work we have used live cell imaging to demonstrate the importance of key residues in the Scp1 actin interface. We have defined two actin binding domains within Scp1 that allow the protein to both bind and bundle actin without the need for dimerization. Green fluorescent protein-tagged mutants of Scp1 also indicate that actin localization does not require the putative phosphorylation site Ser-185 to be functional. Deletion of SCP1 has few discernable effects on cell growth and morphology. However, we reveal that scp1 deletion is compensated for by up-regulation of Sac6. Furthermore, Scp1 levels are increased in the absence of sac6. The presence of compensatory pathways to up-regulate Sac6 or Scp1 levels in the absence of the other suggest that maintenance of sufficient bundling activity is critical within the cell. Analysis of cortical patch assembly and movement during endocytosis reveals a previously undetected role for Scp1 in movement of patches away from the plasma membrane. Additionally, we observe a dramatic increase in patch lifetime in a strain lacking both sac6 and scp1, demonstrating the central role played by actin-bundling proteins in the endocytic process.

Received for publication, December 19, 2007 , and in revised form, April 7, 2008.

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^* Funding for this research was from Wellcome Grant 07479/Z/04/Z (to E. G. A.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. 1 and Tables 1, 2, and 3.

¹ Recipient of a Darwin Trust of Edinburgh studentship.

² Recipient of a Biotechnology and Biological Sciences Research Council, Swindon, United Kingdom studentship.

³ Funded by Biotechnology and Biological Sciences Research Council, Swindon, United Kingdom Grant BB/C510091/1.

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⁴ A Medical Research Council Senior non-clinical fellow (G117/394). To whom correspondence should be addressed. Fax: 44-114-222-2800; E-mail: k.ayscough{at}sheffield.ac.uk.

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