Substrate adapter for ufmylation, the covalent attachment of the ubiquitin-like modifier UFM1 to substrate proteins, in response to endoplasmic reticulum stress (PubMed:23152784, PubMed:30635284). Negatively regulates NF-kappa-B-mediated gene transcription through the control of RELA phosphorylation (PubMed:17785205, PubMed:20228063). Probable tumor suppressor initially identified as a CDK5R1 interactor controlling cell proliferation (PubMed:12054757, PubMed:12737517). Also regulates mitotic G2/M transition checkpoint and mitotic G2 DNA damage checkpoint (PubMed:15790566, PubMed:19223857). Through its interaction with CDKN2A/ARF and MDM2 may induce MDM2-dependent p53/TP53 ubiquitination, stabilization and activation in the nucleus, thereby promoting G1 cell cycle arrest and inhibition of cell proliferation (PubMed:16173922). May also play a role in the rupture of the nuclear envelope during apoptosis (PubMed:23478299). May regulate MAPK14 activity by regulating its dephosphorylation by PPM1D/WIP1 (PubMed:21283629). Required for liver development (By similarity). (Microbial infection) May be negatively regulated by hepatitis B virus large envelope protein mutant pre-s2 to promote mitotic entry. Interacts with CDK5R1; competes with CDK5RAP1 and CDK5RAP2 (PubMed:12737517). Interacts with RELA (PubMed:17785205). Interacts with CHEK1; may negatively regulate CHEK1 and thereby stimulate entry into mitosis (PubMed:19223857). Interacts with CDKN2A/ARF and MDM2; forms a ternary complex involved in regulation of p53/TP53 (PubMed:16173922). Interacts with UFL1; the interaction is direct (PubMed:20164180, PubMed:20228063). Interaction with UFL1 anchors CDK5RAP3 in the cytoplasm, preventing its translocation to the nucleus which allows expression of the CCND1 cyclin and progression of cells through the G1/S transition (By similarity). Interacts with DDRGK1 (PubMed:20228063). Interacts with MAPK14 (PubMed:21283629). Interacts with CCNB1 (PubMed:15790566). Interacts with TUBG1; may regulate CDK5RAP3 in mitotic G2/M transition checkpoint (PubMed:21465471). (Microbial infection) Interacts with hepatitis B virus large envelope protein mutant pre-s2; promotes mitotic entry. Colocalizes and associates with microtubules. Ubiquitously expressed (PubMed:12054757, PubMed:10721722). Expressed in heart, brain, placenta, lung, liver, skeletal muscle, kidney and pancreas. Isoform 3 is expressed in kidney, liver, skeletal muscle and placenta (PubMed:12737517). May be phosphorylated by CDK5. Ubiquitinated. Probably triggers proteasomal degradation and is negatively regulated by UFL1. May be ufmylated. Cleaved by caspases early during apoptosis, the resulting peptides may play a role in rupture of the nuclear envelope. Due to an intron retention. Due to intron retention. Belongs to the CDK5RAP3 family.