BackgroundSeptins belong to the GTPase superclass of proteins and have been functionally implicated in cytokinesis and the maintenance of cellular morphology. They are found in all eukaryotes, except in plants. In mammals, 14 septins have been described that can be divided into four groups. It has been shown that mammalian septins can engage in homo- and heterooligomeric assemblies, in the form of filaments, which have as a basic unit a hetero-trimeric core. In addition, it has been speculated that the septin filaments may serve as scaffolds for the recruitment of additional proteins.Methodology/Principal FindingsHere, we performed yeast two-hybrid screens with human septins 1–10, which include representatives of all four septin groups. Among the interactors detected, we found predominantly other septins, confirming the tendency of septins to engage in the formation of homo- and heteropolymeric filaments.Conclusions/SignificanceIf we take as reference the reported arrangement of the septins 2, 6 and 7 within the heterofilament, (7-6-2-2-6-7), we note that the majority of the observed interactions respect the “group rule”, i.e. members of the same group (e.g. 6, 8, 10 and 11) can replace each other in the specific position along the heterofilament. Septins of the SEPT6 group preferentially interacted with septins of the SEPT2 group (p<0.001), SEPT3 group (p<0.001) and SEPT7 group (p<0.001). SEPT2 type septins preferentially interacted with septins of the SEPT6 group (p<0.001) aside from being the only septin group which interacted with members of its own group. Finally, septins of the SEPT3 group interacted preferentially with septins of the SEPT7 group (p<0.001). Furthermore, we found non-septin interactors which can be functionally attributed to a variety of different cellular activities, including: ubiquitin/sumoylation cycles, microtubular transport and motor activities, cell division and the cell cycle, cell motility, protein phosphorylation/signaling, endocytosis, and apoptosis.

The fasciculation and elongation protein 1 (FEZ1) is a mammalian orthologue of the Caenorhabditis elegans protein UNC-76, which is necessary for axon growth in that nematode. In previous studies FEZ1 has been found to interact with protein kinase C, DISC1, the agnoprotein of the human polyomavirus JC virus, and E4B, a U-box-type ubiquitin-protein isopeptide ligase. We reported previously that FEZ1 and its paralogue FEZ2 are proteins that interact with NEK1, a protein kinase involved in polycystic kidney disease and DNA repair mechanisms at the G 2 /M phase of the cell cycle. Here we report the identification of 16 proteins that interact with human FEZ1-(221-396) in a yeast two-hybrid assay of a human fetal brain cDNA library. The 13 interacting proteins of known functions take part either in transcription regulation and chromatin remodeling (6 proteins), the regulation of neuronal cell development (2 proteins) and cellular transport mechanisms (3 proteins) or participate in apoptosis (2 proteins). We were able to confirm eight of the observed interactions by in vitro pull-down assays with recombinant fusion proteins. The confirmed interacting proteins include FEZ1 itself and three transcription controlling proteins (SAP30L, DRAP1, and BAF60a). In mapping studies we found that the C-terminal regions of FEZ1, and especially its coiled-coil region, are involved in its dimerization, its heterodimerization with FEZ2, and in the interaction with 10 of the identified interacting proteins. Our results give further support to the previous speculation of the functional involvement of FEZ1 in neuronal development but suggest further that FEZ1 may also be involved in transcriptional control.FEZ1 (fasciculation and elongation protein -1) was initially identified as a mammalian orthologue of the Caenorhabditis elegans UNC-76 protein, which is necessary for normal axonal outgrowth, bundling, and elongation in this nematode (1). FEZ1 mRNA is expressed abundantly in the rat adult brain and throughout all developmental stages of the brain in mouse embryos (2, 3). The human FEZ1 includes 392 amino acid residues, and its predicted structural organization shows that the protein possesses three glutamine-rich regions and a coiled-coil region (4) (Fig. 3). A mammalian homologue of FEZ1, the protein FEZ2, which shows ubiquitous tissue expression, was described in rat and human and has 48% amino acid sequence identity to FEZ1 (3).Interestingly, FEZ1 was identified as an interacting protein partner in several yeast two-hybrid screens with independent protein baits. The first bait shown to interact with FEZ1 was the regulatory domain of the protein kinase C (PKC) 4 (2). It was further found that FEZ1 is a cellular substrate for phosphorylation through PKC and that phosphorylated FEZ1 promotes neurite extension of PC12 cells in the absence of nerve growth factor. In a second study, using the C terminus of DISC1 (disrupted-in-schizophenia 1) as bait, FEZ1 was also identified as an interacting protein. The DISC1 gene has been implicated as a...