Acts as a negative regulator of G1 to S cell cycle phase progression by inhibiting cyclin-dependent kinases. Inhibitory effects are additive with GADD45 proteins but occurs also in the absence of GADD45 proteins. Acts as a repressor of the orphan nuclear receptor NR4A1 by inhibiting AB domain-mediated transcriptional activity. May be involved in the hormone-mediated regulation of NR4A1 transcriptional activity. May play a role in mitochondrial protein synthesis. Component of the mitochondrial large ribosomal subunit (mt-LSU) (PubMed:28892042, PubMed:25838379, PubMed:25278503). Mature mammalian 55S mitochondrial ribosomes consist of a small (28S) and a large (39S) subunit. The 28S small subunit contains a 12S ribosomal RNA (12S mt-rRNA) and 30 different proteins. The 39S large subunit contains a 16S rRNA (16S mt-rRNA), a copy of mitochondrial valine transfer RNA (mt-tRNA(Val)), which plays an integral structural role, and 52 different proteins (PubMed:23908630, PubMed:25278503, PubMed:25838379). Interacts with GADD45A, GADD45B and GADD45G (PubMed:12716909). Interacts with NR4A1 via the NR4A1 AB domain (PubMed:15459248). Interacts with ATAD3A and ATAD3B (PubMed:22453275). (Microbial infection) Interacts with the human papilloma virus type 16 (HPV 16) minor capsid protein L2. Using N-terminally tagged constructs, has been found in the nucleus (PubMed:12482659). C-terminally tagged constructs are targeted exclusively to mitochondria (PubMed:22453275). This discrepancy may be explained by masking of a potential N-terminal mitochondrial targeting signal by the tag (PubMed:22453275). Widely expressed. Highly expressed in the thyroid gland, heart, lymph nodes, trachea and adrenal tissues. Expressed at lower level in liver skeletal muscle, kidney, pancreas, testis, ovary and stomach. Barely detectable in adrenal adenoma and papillary thyroid cancer. Down-regulated by p53/TP53 in apoptotic cells. Cells overexpressing GADD45GIP1 were more likely to be in G1 and less likely to be in S phase and grow more slowly than control cells. Inhibiting the expression of GADD45GIP1 promotes cell cycle progression. Belongs to the mitochondrion-specific ribosomal protein mL64 family.