SEPTIN7 (septin 7)

Description	The human SEPT7 gene (NM_001011553) is located on the positive strand of chromosome 7. It consists of an open reading frame (ORF) having 1254 nucleotides which encodes 418 amino acids, consisting of a GTP-binding motif. Its size is about 51 kDa. It has a total of 17 exons.
Transcription	Till date three transcript variants encoding different isoforms are known for SEPT7 [Figure1]. These are transcript variant 1 (accession number NM_001788), transcript variant 2 (NM_001011553) and transcript variant 3 (accession number NM_001242956). Transcript variant 1 (TV1) is known to be the longest transcript encoding the longest isoform (1). As compared to TV1, TV2 (transcript variant 2) has no in-frame segment in the 5' coding region giving rise to an isoform (2). It is 1 aa shorter than isoform 1. On the other hand transcript variant (3) also does not possess an in-frame segment in the 5' coding region which results in an isoform (3). This isoform (3) is 36 aa shorter than isoform 1. In fact alternative splicing results in multiple transcript variant. SEPT7 is linked to Papillary thyroid carcinoma (PTC). It was found that SEPT7 mRNA expression was relatively lower expressed in follicular variant of classical type of PTC (FVPTC) as compared to classical type PTC (CVPTC) (Igci et al., 2011)
Pseudogene	There are several related pseudogenes for SEPT7 which have been identified on chromosomes 5, 7, 9, 10, 11, 14, 17 and 19. A novel septin, SEPT13 has been shown to be related to SEPT7 and represent a transcribed pseudogene of SEPT7 (Hall et al., 2005).

Description	SEPT7 belongs to the septin family that is known to be a highly conserved subfamily of GTPases and is well recognized as a novel component of the cytoskeleton. It consists of an open reading frame (ORF) having 1254 nucleotides which encodes 418 amino acids, consisting of a GTP-binding motif (Nakatsuru et al., 1994). The amino acid sequence of SEPT7 protein is shown in Figure 2. Septins are GTP-binding proteins which are known to contain all the necessary elements to perform the general GDP-to-GTP conformational switch. Sept7, the only member of the Sept7 subgroup, forms a very tight G-interface dimer in the GDP-bound state. It has been shown that the stability of the interface is dramatically decreased by exchanging GDP with a nucleoside triphosphate, which is believed to influence filament formation and dynamics via Sept7 (Zent & Wittinghofer, 2014). SEPT7 is having a molecular mass of approximately 51 kDa and has a basal isoelectric point of 8.76. The protein encoded by this gene has a high similarity to the CDC10 protein of Saccharomyces cerevisiae. It also shows similarity to Diff 6 of Drosophila and H5 of mouse. All of these similar proteins contain a GTP-binding motif. The GTPase activity of SEPT7 is required for filament formation. It is also required for the association of centromere-associated protein E (CENPE) with the kinetochore. Multiple transcript variants are formed as a result of alternative splicing. This protein has a role in ciliogenesis and collective cell movements. The GTPase domain of SEPT7 is unique as it contains a conserved sequence near its end - the Septin Unique Element (SUE) (Pan et al., 2007, Steels et al., 2007, Versele et al., 2004). SEPT7 has additional conserved elements that flank the GTPase domain. It possesses a polybasic region at N-terminal to the GTPase domain that has been shown to bind phosphoinositides (Casamayor & Snyder, 2003, Zhang et al., 1999). In fact, mammalian SEPT7 exist solely as 6- to 8-unit heteromeric complexes within the cell (Sellin et al., 2011). In mammals, SEPT7 forms a complex with SEPT2, SEPT4 and SEPT6 which was first identified in brain tissue (Hsu et al., 1998). SEPT7 monomers within the unit complex interact through G-G and NC-NC interfaces, thereby pairing up with other septin members. These G-G and NC-NC interactions alternate along the unit complex, and are necessary for its formation (Sirajuddin et al., 2007). SEPT7 also shows non-septin interactions. 9% of the interactors have been identified to be non-septins for SEPT7 (Nakahira et al., 2010). SEPT7 was identified as a mutual partner with SEPT9 at a very high percentage (Nakahira et al., 2010). SEPT7 is known to be expressed in several tissues. It is therefore believed to be much more involved in processes like cell division. It does belong to the group of septins with the least members (SEPT7 and 13, only) and is found in almost all of the heterofilaments known till date. It is therefore a basic element for filament formation (Nakahira et al., 2010)
Expression	SEPT7 has cytoplasmic expression in most tissues but is always found associated to actin filaments as well as several regions through-out mitosis (Zhu et al., 2008). There are already cytoplasmic, nuclear, and overall (total) SEPT7 protein expression levels which have also been determined in PTC (Igci et al., 2014). SEPT7 was also shown to be negatively regulated by miR-30a-5p in glioma cells (Jia et al., 2013). There is very high expression of SEPT7 proteins in cerebral cortex but moderate expression in hippocampus, lateral ventricle and cerebellum of CNS. SEPT7 is also moderately expressed in gall bladder, pancreas, stomach, colon, kidney, testis, prostate, breast, thyroid gland. It has low expression in liver, small intestine, bone marrow and lung. But it has not been detected in skeletal muscles, smooth muscles and heart muscles.
Localisation	SEPT7 is distributed throughout the cytoplasm in prometaphase cells. During metaphase it is associated with the spindle. While at anaphase it is associated with the central spindle and at the cleavage furrow in anaphase cells. It has been detected at the midbody in telophase and is associated with actin stress fibers (Zhu et al., 2008).
Function	SEPT7 has cytoskeletal GTPase which is responsible for filament-forming. It is needed for normal orientation of the actin cytoskeleton. It is needed for normal progress through mitosis. It is also involved in cytokinesis. It has been revealed that septins are extremely necessary for cytokinesis in fibroblasts, but of relatively little significance in cells of the hematopoietic system by studying genetic loss of the septin subunit SEPT7 in vivo (Menon et al., 2014). SEPT7 is of utmost requirement for associating centromere-associated protein E (CENPE) with the kinetochore. It is known to play a role in ciliogenesis and collective cell movements. SEPT7 is a GTP-binding protein that is known to form hetero-oligomeric complexes as well as higher-order structures for example filaments and rings (Mostowy & Cossart, 2012). It has been shown that Septin 7 is required for axonal association of Schwann cells (Roth et al., 2013). It also helps in glucose transporter trafficking. Septin 7 regulates glucose transporter trafficking by forming a complex with CD2AP and nephrin (Wasik et al., 2012). It is known to function in gliomagenesis as well as in suppression of glioma cell growth (Jia et al., 2010).
Homology	SEPT7 is highly similar to the CDC10 (cell division cycle 10) protein of Saccharomyces cerevisiae. A novel human gene (418-amino acid product) has been cloned which shared almost 39% and 54% sequence identity with yeast CDC10 and mouse H5 proteins, respectively (Nakatsuru et al., 1994). This protein is also similar to Diff 6 of Drosophila to a good extent. Each of these known sequence homologues contains a common GTP-binding motif.

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