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 ribosome large subunit (39S) which comprises a 16S rRNA and about 50 distinct proteins. Interacts with GADD45A, GADD45B and GADD45G. Interacts with NR4A1 via the NR4A1 AB domain. Interacts with ATAD3A and ATAD3B. Using N-terminally tagged constructs, has been found in the nucleus. C-terminally tagged constructs are targeted exclusively to mitochondria. This discrepancy may be explained by masking of a potential N-terminal mitochondrial targeting signal by the tag. Belongs to the mitochondrion-specific ribosomal protein mL64 family.