The Homer family proteins

Abstract

The Homer family of adaptor proteins consists of three members in mammals, and homologs are also known in other animals but not elsewhere. They are predominantly localized at the postsynaptic density in mammalian neurons and act as adaptor proteins for many postsynaptic density proteins. As a result of alternative splicing each member has several variants, which are classified primarily into the long and short forms. The long Homer forms are constitutively expressed and consist of two major domains: the amino-terminal target-binding domain, which includes an Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) homology 1 (EVH1) domain, and the carboxy-terminal self-assembly domain containing a coiled-coil structure and leucine zipper motif. Multimers of long Homer proteins, coupled through their carboxy-terminal domains, are thought to form protein clusters with other postsynaptic density proteins, which are bound through the amino-terminal domains. Such Homer-mediated clustering probably regulates or facilitates signal transduction or cross-talk between target proteins. The short Homer forms lack the carboxy-terminal domain; they are expressed in an activity-dependent manner as immediate-early gene products, possibly disrupting Homer clusters by competitive binding to target proteins. Homer proteins are also involved in diverse non-neural physiological functions.

Figure 1

Primary structures of Homer family proteins. The conserved amino-terminal EVH1-like domain (which shows 80% sequence similarity between family members) is in yellow. The conserved region of Homer1 (CRH1) [19] and a proline motif (P-motif, 138-Ser-Pro-Leu-Thr-Pro-142) is specific to the mammalian Homer1 subfamily. The carboxy-terminal regions contain coiled-coil and leucine zipper structures and show only 30% sequence similarity among the family members. The coiled-coil regions are in orange, green and pink for the Homer1, Homer2, and Homer3 alternatively spliced forms, respectively. The leucine zipper structures, as predicted by Sun et al. [16], are shown as LzipA and LzipB in gray. The nomenclature is from Soloviev et al. [9], Saito et al. [10], Bottai et al. [4] and Klugmann et al. [11]. Homer3a_xx and Homer3b_xx represent the products of four alternative splicing variants, where xx can be 00, 01, 10, or 11 to show the combination of two three-amino-acid insertions (purple) in the coiled-coil domain, as has been suggested for the human forms [9]. Residues involved in ligand contacts are colored light blue.

Figure 2

A phylogenic tree of Homer family proteins. Whole protein sequences of the longest isoform of each family member from human (Homo sapiens), chimp (Pan troglodytes), dog (Canis familiaris), rat (Rattus norvegicus), mouse (Mus musculus), chicken (Gallus gallus), frog (Xenopus laevis), Fugu (Takifugu rubripes), zebrafish (Danio rerio), fly (Drosophila melanogaster) and mosquito (Anopheles gambiae) were aligned. The accession numbers of the proteins are indicated in brackets. Multiple sequence alignment was performed using CLUSTAL X [70]. Phylogenetic analysis was constructed using the neighbor-joining method [71] using PAUP* version 4.0 beta [72], and the reliability of the tree was estimated by bootstrapping. The tree was rooted with proteins from invertebrates (fly and mosquito). The branch lengths are proportional to the amount of inferred evolutionary change, and numbers between internal nodes indicate bootstrap values as percentages of 1,000 replications.

Figure 3

The crystal structure of the Homer EVH1 domain [17]. (a) Ribbon diagrams of the Homer1 EVH1 domain, with five residues of mGluR1 (residues 1,146-1,150) as a representative bound ligand in red. The side chains of the adjacent serine and proline in the bound ligand are oriented away from the Homer surface, and the phenylalanine residue forms a unique contact that is not shared by other EVH1 domains (not shown). (b) A surface representation of the Homer EVH1 domain, showing the location of residues that are essential (red) and nonessential (light blue) for binding to ligand (dark blue). Reproduced with permission from [17].

Figure 4

Homer proteins form a physical tether linking signalingmolecules in postsynaptic densities. (a) Cultured hippocampal neurons co-immunostained for Homer2a/b (green) and the neuronal dendritic marker MAP2 (red). (b) Dendritic spines of cultured hippocampal neurons expressing exogenous GFP-Homer2 (green) and immunostained for synaptophysin (a presynapse marker; red). (c) A model of the long Homer multimer-mediated postsynaptic protein complex and Homer1a. Long Homer forms (Homers) bind to each other through their carboxy-terminal domains (probably forming a tetramer [20]) and to the target proteins, such as mGluR1α/5, IP3 receptor, NMDA receptor and Drebrin, to the actin cytoskeleton through their amino-terminal domains, forming a cluster at the postsynaptic density area. Homer1a, which lacks the carboxy-terminal domain, is thought to compete with long Homer forms for binding to target proteins, thus disrupting the cluster. For instance, Homers modulate mGluR-induced intracellular calcium release by linking mGluRs and IP₃R: activation of mGluRs results in the phospholipase C (PLC)-mediated hydrolysis of membrane phosphatidylinositol diphosphate (PIP₂) to diacylglycerol (DAG) and IP₃, which activates the IP₃R to release intracellular calcium. Abbreviations: AMPAR, 5-methyl-4-isoxazolepropionate receptor; CC-LZ, carboxy-terminal coiled-coil structure and leucine zipper motifs of long Homer forms; ER, endoplasmic reticulum; EVH1, the amino-terminal EVH1 domain of Homer; G, G protein; GKAP, guanylate kinase-associated protein; GRIP, glutamate receptor interacting protein; mGluR, metabotropic glutamate receptor; NMDAR, N-methyl-D-aspartate receptor; VDCC, voltage-dependent Ca²⁺ channel.