IRSp53: crossing the road of membrane and actin dynamics in the formation of membrane protrusions Giorgio Scita 1, 2 , Stefano Confalonieri 1 , Pekka Lappalainen 3 and Shiro Suetsugu 4 1 IFOM (FIRC Institute for Molecular Oncology) - Foundation, Via Adamello 16, 20139 Milan, Italy 2 School of Medicine, University of Milan, 20122 Milan, Italy 3 Institute of Biotechnology, University of Helsinki, P.O. Box 56, 00014 Helsinki, Finland 4 Laboratory of Membrane and Cytoskeleton Dynamics at the Institute of Molecular and Cellular Biosciences, the University of Tokyo and PRESTO (Precursory Research for Embryonic Science and Technology), Japan Science and Technology Agency, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan A tight spatiotemporal coordination of the machineries controlling membrane bending and trafficking, and actin dynamics is crucial for the generation of cellular protru- sions. Proteins that are simultaneously capable of regulating actin dynamics and sensing or inducing mem- brane curvature are predicted to have a prominent role. A prototypical example of this type of proteins is the insulin receptor tyrosine kinase substrate of 53 kDa, the founding member of a recently discovered family of proteins, including missing-in-metastasis and ABBA (actin-bundling protein with BAIAP2 homology). Struc- tural, biochemical and cell biological experiments sup- port the unique role of this family as transducers of signalling, linking the protruding membrane to the underlying actin cytoskeleton. Introduction Cell migration is a remarkably complex process, which relies on the sophisticated decoding of signals in a 3D setting. This is achieved by a tight spatial and temporal coordination of the machineries controlling actin polymer- ization and trafficking of membrane and membrane-bound proteins. Dynamic actin filament turnover is generally regarded as the means to generate forces for motility [1,2]. Less established is the function of intracellular traf- ficking of membrane and signalling proteins, which can contribute to the acquisition of migratory behaviours in various ways. Accordingly, several possible links between plasma membrane trafficking and actin-based cell move- ment have been proposed These include the delivery of adhesion and signalling molecules to the front of the cells, polarization of the plasma membrane and the generation of membrane flow, which has been proposed as an alternative force generator to propel a cell forward [3], although its existence in certain cells has been disputed [4]. Addition- ally, more recent evidence suggested that internalization and recycling of membrane might also be necessary for a moving cell to adjust the cell surface area continually to match changes in shape [5]. Finally, a wealth of genetic experiments revealed that intracellular trafficking is crucial to promote spatial restriction of signalling, confining and polarizing key actin regulatory molecules and generating self-controlled feedback loops that ensure univectorial production of actin-based motility forces. These, in turn, are required for the extension of various types of migratory cell protrusions [6–8], such as filopodia and lamellipodia. Within this context, a prominent role is predicted to be exerted by proteins that have a dual role in membrane dynamics and in the actin cytoskeleton, which contribute to actin dynamics by controlling the architectural organ- ization of the actin filament meshwork, and sense mem- brane curvature. A prototypical example of this type of protein is insulin receptor tyrosine kinase substrate of 53 kDa (IRSp53), also known as the binding partner of the brain-specific angiogenesis inhibitor 1-associated protein 2 (BAIAP2) [9–11], and its family members, whose emerging structural, biochemical and cell biological functions sup- port their unique role as transducers of signalling linking the deformation of the plasma membrane caused by pseu- dopod extension, with the underlying actin cytoskeleton. Here, we review the most recent evidence illustrating the molecular mechanisms through which IRSp53 might func- tion in regulating the extension of different types of migratory protrusions in response to distinct signalling pathways. The IRSp53 family of proteins: a structural outlook IRSp53 is the founding member of a family of proteins featuring the presence of the so-called IRSp53 and missing- in-metastasis (MIM) [12] homology domain, hereafter referred to as the IRSp53–MIM domain (IMD). This domain, which belongs to the larger family of the Bin– amphipysin–Rvs167 (BAR) domain (see later), binds to both actin and lipids. Members of this family can be sub- divided in two groups, the IRSp53-like and MIM-like members (Figure 1a). The former, which is the focus here, are invariably characterized by a domain organization typical of signalling proteins that includes in addition to IMD a canonical SH3 domain plus variable protein– protein interaction surfaces (Figure 1a). The MIM group includes two genes, MIM and ABBA [13]. These genes, at Review Corresponding author: Scita, G. (giorgio.scita@ifom-ieo-campus.it). 52 0962-8924/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2007.12.002 Available online 22 January 2008