SIP (CacyBP/SIP) was originally identified in Ehrilich ascities tumor cells. SIP has been shown to interact with a variety of ligands including S100, Skp1, tubulin, Siah-1, actin and ERK1/2. Studies show that binding with tubulin is essential for cytoskeleton rearrangement during the differentiation of NB2a cells. SIP interacts with ERK1/2 and will lead to the ELK-1 activation and plays an important role in cell differentiation. SIP has also been found to contain phosphatase activity recently and target ERK1/2. E217K mutant will lead to the drop of phosphatase activity toward ERK1/2, which suggests that the C-terminal domain acts as a phosphatase like domain (1).

Reference
1. Kilanczyk, E., Filipek, S. and Filipek, A. (2011) ERK1/2 is dephosphorylated by a novel phosphatase--CacyBP/SIP. Biochem Biophys Res Commun, 404, 179-183. PMID: 21110948

    RPAP2, also named RNA Pol II-associated protein, can interact with Pol II. Analysis of human RPAP2, S. cerevisiae and S. pombe homolog Rtr1 reveals an evolutionary conserved domain (DUF408) located at N-terminus of the proteins with diverse sequence outside of this region. Studies show that RPAP2 can interact with Rpb1, the largest subunit of Pol II, and mediate the dephosphorylation of Ser7 on CTD of Pol II. The DUF408 domain is involved in establishment of protein-protein interaction (1).

Reference
1. Egloff, S., Zaborowska, J., Laitem, C., Kiss, T. and Murphy, S. (2012) Ser7 phosphorylation of the CTD recruits the RPAP2 Ser5 phosphatase to snRNA genes. Mol Cell, 45, 111-122. PMID: 22137580