Microtubule +TIPs at a glance

Abstract

Microtubules are highly dynamic hollow tubes that are involved in many vital cellular activities, including maintenance of cell shape, division, migration and intracellular transport. They are assembled from heterodimers of α- and β-tubulin that align in a head-to-tail fashion. Microtubules are, thus, intrinsically polar because they contain two structurally distinct ends: a slow-growing minus end, exposing α-tubulin subunits; and a fast-growing plus end, exposing β-tubulin subunits (for a review, see Nogales and Wang, 2006). In mammalian cells, microtubule minus ends are often stably anchored, whereas the plus ends are highly dynamic and stochastically switch between phases of growth and shrinkage, a process that is powered by GTP hydrolysis.Microtubule plus-end tracking proteins (+TIPs) are a structurally and functionally diverse group of proteins that are distinguished by their specific accumulation at microtubule plus ends (Mimori-Kiyosue et al., 2000; Perez et al., 1999; Schuyler and Pellman, 2001). +TIPs typically target growing but not shrinking microtubule ends; however +TIP association with depolymerizing ends can occur and, in some organisms such as budding yeast, is even quite common. In this Cell Science at a Glance article we review and illustrate the current knowledge of these peculiar proteins, summarize their structural and functional properties, and discuss the proposed molecular mechanisms that they use to track microtubule ends.The first reported +TIP was cytoplasmic linker protein of 170 kDa (CLIP-170, officially known as CLIP1) (Perez et al., 1999). Since its discovery, more than 20 different +TIP families have been identified. +TIPs are usually multidomain and/or multisubunit proteins that range in size from a few hundred up to thousands of residues. They can be cytoplasmic or membrane bound, and comprise motor and non-motor proteins (for a review, see Akhmanova and Steinmetz, 2008). +TIPs can be classified on the basis of prominent structural elements that enable them to interact with each other and with microtubules; however, in some cases, +TIPs combine features characteristic of several +TIP classes.End-binding (EB) family proteins contain a highly conserved N-terminal domain that adopts a calponin homology (CH) fold (Korenbaum and Rivero, 2002) and is responsible for microtubule binding (Hayashi and Ikura, 2003). In mammalian EB1 and EB3, a CH domain with the adjacent linker sequence is sufficient for plus-end tracking (Komarova et al., 2009; Skube et al., 2010); however dimerization is important for microtubule plus-end recognition by their yeast homologue Bim1 (Zimniak et al., 2009). The C terminus of EB proteins harbors an α-helical coiled-coil domain that mediates parallel dimerization of EB monomers (Honnappa et al., 2005; Slep et al., 2005). It further comprises the unique EB homology (EBH) domain and an acidic tail encompassing a C-terminal EEY/F motif, reminiscent of those of α-tubulin and CLIP-170 (Komarova et al., 2005; Miller et al., 2006; Weisbrich et al., 2007). Notably, plant EB proteins lack the EEY/F motif, and some EB family members, such as EB1c in Arabidopsis thaliana, exhibit a positively charged C-terminus that is responsible for nuclear localization (Komaki et al., 2010). Both the EBH domain and the EEY/F motif enable the EB proteins to physically interact with an array of +TIPs to recruit them to microtubule ends.The cytoskeleton-associated protein glycine-rich (CAP-Gly) domain is a small globular module that contains a unique conserved hydrophobic cavity and several characteristic glycine residues (Li et al., 2002; Saito et al., 2004). CAP-Gly domains use their hydrophobic cavity to confer interactions with microtubules and EB proteins by specifically recognizing C-terminal EEY/F sequence motifs (Honnappa et al., 2006; Mishima et al., 2007; Weisbrich et al., 2007). Prominent examples are the CLIP proteins and the large subunit of the dynactin complex p150glued. A single CAP-Gly domain of CLIP-170, together with the adjacent serine-rich region, can track growing microtubule ends (Gupta et al., 2009).The largest group of +TIPs comprises large and complex, often multidomain, proteins containing low-complexity sequence regions that are rich in basic, serine and proline (basic-S/P) residues. They share the small four-residue motif Ser-x-Ile-Pro (SxIP, where x denotes any amino acid), which is specifically recognized by the EBH domain of EB proteins (Honnappa et al., 2009). Prominent examples of this diverse class of +TIPs are the adenomatous polyposis coli (APC) tumour suppressor, the spectraplakin microtubule–actin crosslinking factor (MACF) and the mitotic centromere-associated kinesin (MCAK). Because SxIP motifs are very short, they can be easily acquired or lost during evolution; for example, CDK5RAP2, a protein implicated in microcephaly, contains an EB1-binding SxIP motif in humans and dogs but not in rodents (Fong et al., 2009).Proteins with TOG or TOG-li