Substrate adapter for ufmylation, the covalent attachment of the ubiquitin-like modifier UFM1 to substrate proteins, in response to endoplasmic reticulum stress (By similarity). Negatively regulates NF-kappa-B-mediated gene transcription through the control of RELA phosphorylation. Probable tumor suppressor initially identified as a CDK5R1 interactor controlling cell proliferation. Also regulates mitotic G2/M transition checkpoint and mitotic G2 DNA damage checkpoint. 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. May also play a role in the rupture of the nuclear envelope during apoptosis. May regulate MAPK14 activity by regulating its dephosphorylation by PPM1D/WIP1 (By similarity). Required for liver development (By similarity). Interacts with CDK5R1; competes with CDK5RAP1 and CDK5RAP2 (PubMed:10721722). Interacts with RELA (By similarity). Interacts with CHEK1; may negatively regulate CHEK1 and thereby stimulate entry into mitosis (By similarity). Interacts with CDKN2A/ARF and MDM2; forms a ternary complex involved in regulation of p53/TP53 (By similarity). Interacts with UFL1; the interaction is direct (PubMed:20531390). 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 (PubMed:20531390). Interacts with DDRGK1 (By similarity). Interacts with MAPK14 (By similarity). Interacts with CCNB1 (By similarity). Interacts with TUBG1; may regulate CDK5RAP3 in mitotic G2/M transition checkpoint (By similarity). Colocalizes and associates with microtubules. Expressed in vascular endothelium. Up-regulated in failing heart. Highly expressed in the ventricular section in subacute and chronic ischemic heart failure. May be phosphorylated by CDK5. May be ufmylated. Ubiquitinated. Probably triggers proteasomal degradation and is negatively regulated by UFL1. Cleaved by caspases early during apoptosis, the resulting peptides may play a role in rupture of the nuclear envelope. Belongs to the CDK5RAP3 family.