Secreted protein acidic and rich in cysteine (SPARC), also known as Osteonectin (ON), is a member of the SPARC family. SPARC consists of three domains: an EF-hand domain, a follistatin-like domain and a Kazal-like domain, and each of which has independent activity and unique properties. The activity of SPARC is context-and cell-type-dependent, which is highlighted by the fact that SPARC has shown seemingly contradictory effects on tumor progression in both clinical correlative studies and in animal models. The location of SPARC in the nuclear matrix of certain proliferating cells, but only in the cytosol of postmitotic neurons, indicates potential functions of SPARC as a nuclear protein, which might be involved in the regulation of cell cycle progression and mitosis. It functions not only to modulate cell-cell and cell-matrix interactions, but its de-adhesive and growth inhibitory properties in non-transformed cells have led to studies to assess its role in cancer. Its divergent actions reflect the complexity of this protein, because in certain types of cancers, such as melanomas and gliomas, SPARC is associated with a highly aggressive tumor phenotype, while in others, mainly ovarian, neuroblastomas and colorectal cancers, SPARC may function as a tumor suppressor. Recent studies have also demonstrated a role for SPARC in sensitizing therapy-resistant cancers. Notably, SPARC is linked to human obesity.
Secreted protein acidic and rich in cysteine (SPARC), also known as Osteonectin (ON), is a member of the SPARC family. SPARC consists of three domains: an EF-hand domain, a follistatin-like domain and a Kazal-like domain, and each of which has independent activity and unique properties. The activity of SPARC is context- and cell-type-dependent, which is highlighted by the fact that SPARC has shown seemingly contradictory effects on tumor progression in both clinical correlative studies and in animal models. The location of SPARC in the nuclear matrix of certain proliferating cells, but only in the cytosol of postmitotic neurons, indicates potential functions of SPARC as a nuclear protein, which might be involved in the regulation of cell cycle progression and mitosis. It functions not only to modulate cell-cell and cell-matrix interactions, but its de-adhesive and growth inhibitory properties in non-transformed cells have led to studies to assess its role in cancer. Its divergent actions reflect the complexity of this protein, because in certain types of cancers, such as melanomas and gliomas, SPARC is associated with a highly aggressive tumor phenotype, while in others, mainly ovarian, neuroblastomas and colorectal cancers, SPARC may function as a tumor suppressor. Recent studies have also demonstrated a role for SPARC in sensitizing therapy-resistant cancers. Notably, SPARC is linked to human obesity.
Striated muscle preferentially expressed protein kinase, also known as aortic preferentially expressed protein 1, APEG-1, SPEG and KIAA1297, is a protein which belongs to theprotein kinase superfamily and CAMK Ser/Thr protein kinase family. SPEG / APEG-1 contains twofibronectin type-III domains, nine Ig-like (immunoglobulin-like) domains, two protein kinase domains. Isoform1of SPEG is preferentially expressed in striated muscle. Non-kinase form such as isoform3of SPEG is predominantly expressed in the aorta. Isoform3of SPEG appears to be expressed only in highly differentiated ASMC in normal vessel walls and down-regulated in dedifferentiated ASMC. Isoform3of SPEG may have a role in regulating the growth and differentiation of arterial smooth muscle cells. Isoform3of SPEG is quickly down-regulated in response to vascular injury, when ASMC cells change from a quiescent to a proliferative phenotype.
Astrin is a homodimeric protein with a globular head domain and a long stalk. It is an essential component of the mitotic spindle. Astrin is highly expressed in testis and is detected at low levels in placenta, liver, pancreas, thymus and colon. Its expression is cell cycle-regulated, with an increase from prophase to cytokinesis and return to basal levels at the next G1 phase. In non-dividing cells Astrin is associated with microtubules unless under cellular stress, then it is localized in stress granules.
Sperm Equatorial Segment Protein 1 (SPESP1) is a member of the SPESP1 family. SPESP1 is highly expressed in the testis, where it is localized to the acrosome of postmeiotic stages of spermiogenesis; it is expressed at lower levels in the placenta and fetal lung. SPESP1 is involved in the multicellular organisimal development. Disruption of SPESP1 leads to abnormal distribution of sperm proteins resulting in a detached membrane from the equatorial segment and less fertile sperm. SPESP1 may interact with IZUMO1 and MN9 antigen and it contains an N-glycosylation site as well as several cAMP-dependent kinase, protein kinase C, and casein kinase II consensus phosphorylation sites.
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