GCIP, or Grap2 and cyclin-D-interacting protein (also Human homologue of murine maternal Id-like molecule (HHM)), is found in eukaryotes, and in the protein Swiss:O95273, residues 149-190 constitute a helix-loop-helix domain, residues 190-240 an acidic region, and 240-261 a leucine zipper domain. GCIP interacts with full-length Grap2 protein and with the COOH-terminal unique and SH3 domains (designated QC domain) of Grap2. It is potentially involved in the regulation of cell differentiation and proliferation through Grap2 and cyclin D-mediated signalling pathways [1]. In mice, it is involved in G1/S-phase progression of hepatocytes, which in older animals is associated with the development of liver tumours. In vitro it acts as an inhibitory HLH protein, for example, blocking transcription of the HNF-4 promoter. In its function as a cyclin D1-binding protein it is able to reduce CDK4-mediated phosphorylation of the retinoblastoma protein and to inhibit E2F-mediated transcriptional activity [2]. GCIP has also been shown to have interact physically with Rad (Ras associated with diabetes), Rad being important in regulating cellular senescence [3]. GCIP adopts a V-shaped conformation, with N and C-terminal five-helix bundles. This entry represents the N-terminal domain [4].

1: GCIP, a novel human grap2 and cyclin D interacting protein, regulates E2F-mediated transcriptional activity. Xia C, Bao Z, Tabassam F, Ma W, Qiu M, Hua S, Liu M; J Biol Chem. 2000;275:20942-20948. PMID:10801854

2: Maid (GCIP) is involved in cell cycle control of hepatocytes. Sonnenberg-Riethmacher E, Wustefeld T, Miehe M, Trautwein C, Riethmacher D; Hepatology. 2007;45:404-411. PMID:17256742

3: A novel senescence-evasion mechanism involving Grap2 and Cyclin D interacting protein inactivation by Ras associated with diabetes in cancer cells under doxorubicin treatment. Lee I, Yeom SY, Lee SJ, Kang WK, Park C; Cancer Res. 2010;70:4357-4365. PMID:20460530

4: Structure of a dominant-negative helix-loop-helix transcriptional regulator suggests mechanisms of autoinhibition. Ishii R, Isogaya K, Seto A, Koinuma D, Watanabe Y, Arisaka F, Yaguchi S, Ikushima H, Dohmae N, Miyazono K, Miyazawa K, Ishitani R, Nureki O; EMBO J. 2012 May 30; 31(11): 2541–2552. PMID:22453338