Actin and an actin-binding protein, tropomyosin, usually associated in a range of cytoplasmic structures, were shown to be associated with the chromosome scaffold in interphase and metaphase chromosomes and function in combination to aid in chromosome compaction. The kinetochore region that anchors the microtubules during spindle formation is also thought to be a part of the scaffolding network. Licensing Factor Model-1 (LFM-1) was found to distribute along the axis and the external surface of the chromatids and is thought to play a role in cell cycle-dependent events. More recently, other components of the chromosome scaffold have been identified. The fluorescently labeled scaffolds appear helically coiled, and the coiling of sister chromatid scaffolds displays a mirror symmetry. SMC2 has been localized to the center of the long axis of the chromatid arms of mitotic metaphase chromosomes by immunofluorescence and has been shown to be very loosely associated with interphase nuclei. ScII, another prominent member of the mitotic chromosome scaffold, is a structural maintenance of chromosomes (SMCs)-type protein, which is now also called SMC2. The first and major part of the chromosome scaffold protein network to be conclusively identified was ScI, identical to topoisomerase II (topo II), which was found to be concentrated in the axial region of the expanded mitotic chromosomes but absent in the loop domains.
Two major scaffold proteins (Sc proteins) have been identified, ScI and ScII. The composition of the chromosome scaffold differs based on the species and the developmental stage of the organism, but is predominantly comprised of nonhistone proteins in low abundance (5–10%) forming a structural framework that defines the shape of the chromosome. Sridharan, in Brenner's Encyclopedia of Genetics (Second Edition), 2013 Scaffold-Associated Proteins
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