Structural maintenance of chromosomes protein 4 (SMC-4) also known as chromosome-associated polypeptide C (CAP-C) or XCAP-C homolog is a protein that in humans is encoded by the SMC4gene.[5][6][7] SMC-4 is a core subunit of condensin I and II, large protein complexes involved in high order chromosome organization,[8] including condensation and segregation.[9] SMC-4 protein is commonly associated with the SMC-2 protein, another protein complex within the SMC protein family. SMC-4 dimerizes with SMC-2, creating the flexible and dynamic structure of the condensin holocomplex.[8] An over-expression of the SMC-4 protein is shown to impact carcinogenesis.[10][11][9]
Structure of a condensin protein holocomplex, displaying the SMC-4/SMC-2 heterodimer, and additional subunits. Kleisin is also depicted (blue).
The primary 5 domain structure of SMC proteins is highly conserved among species. The basic structure of SMC proteins are characterized by a non-helical hinge group, separated by two anti-parallel α-helical coiled-coil domains, along with two Amino-terminal globular domains containing ATP hydrolytic sites, or nucleotide-binding motifs located at the C-terminus and N-terminus called the Walker A and Walker B motifs.[12]
Ineukaryotes, dimerization is mediated by the self-folding of the non-helical hinge group on the SMC protein. Dimerization occurs at the non-helical hinge group of SMC-4, which then associates with the non-helical hinge group of SMC-2, creating a V-shaped heterodimeric structure. the holocomplex of condensin contains the SMC-4 and SMC-2 heterodimer subunits, along with 3 other non-SMC subunits, CAP-D2, CAP-G, and CAP-H.[9]
In the condensin holocomplex, a protein subunit called kleisin joins the C-terminus and N-terminus ATPase end domains of both SMC-4 and SMC-2 proteins. when the condensin holocomplex is bound with ATP at these end domains, the condensin will assume a "closed" conformation state.[8] SMC-4 is a dynamic and flexible protein, allowing different domain components to occasionally interact with others. This is speculated to be involved in the mechanical ability of the complex when associated with chromosomes.[8] In budding yeast, these interactions may result in open "O" appearances, or collapsed B-shaped states as a result of its dynamic ability.[13]
The SMC-4 protein is associated with abnormal cell and tumor growth, and involved with migration and invasion. In general, the presence of over-expressed SMC-4 proteins is thought to be correlated with carcinogenesis.[10]
It is found that an over-expression or down-regulation of the SMC-4 protein alters TGFβ/Smad signaling pathways in glioma cells. SMC-4-transduced glioma cells showed activation of the TGFβ/Smad signaling pathway which was not present in SMC-4 silenced glioma cells. This pathway was shown to be correlated with an "aggressive" behavioral phenotype in glioma cells. An over-expression of SMC-4 can induce a higher rate of proliferation, and ultimately increased invasive capability. A down-regulation of SMC-4 reduced this quality.[10]
The SMC-4 protein is involved with normal lung development however, adenocarcinoma lung tissue shows an over-expression of SMC-4. additionally, SMC-4 may act as independent prognostic factor for carcinogenesis and lung adenocarcinoma.[9]
Studies suggest that over-expression of the SMC-4 protein in human liver tissue may be correlated with progression of hepatocellular carcinoma.[11]
^Ryu JK, Katan AJ, van der Sluis EO, Wisse T, de Groot R, Haering CH, Dekker C (December 2020). "The condensin holocomplex cycles dynamically between open and collapsed states". Nature Structural & Molecular Biology. 27 (12): 1134–1141. doi:10.1038/s41594-020-0508-3. PMID32989304. S2CID222146992.
Suzuki Y, Yoshitomo-Nakagawa K, Maruyama K, Suyama A, Sugano S (October 1997). "Construction and characterization of a full length-enriched and a 5'-end-enriched cDNA library". Gene. 200 (1–2): 149–156. doi:10.1016/S0378-1119(97)00411-3. PMID9373149.
Maruyama K, Sugano S (January 1994). "Oligo-capping: a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides". Gene. 138 (1–2): 171–174. doi:10.1016/0378-1119(94)90802-8. PMID8125298.
