Substrate-recognition subunit of the CTLH E3 ubiquitin- protein ligase complex that selectively accepts ubiquitin from UBE2H and mediates ubiquitination and subsequent proteasomal degradation of the transcription factor HBP1 (Probable) (PubMed:29911972). Binds proteins and peptides with a Pro/N-degron consisting of an unmodified N-terminal Pro followed by a small residue, and has the highest affinity for the peptide Pro-Gly-Leu-Trp (PubMed:29632410). Binds peptides with an N-terminal sequence of the type Pro-[Ala,Gly]- [Leu,Met,Gln,Ser,Tyr]-[Glu,Gly,His,Ser,Val,Trp,Tyr]. Does not bind peptides with an acetylated N-terminal Pro residue (PubMed:29632410). {ECO:0000269|PubMed:29632410, ECO:0000269|PubMed:29911972, ECO:0000305}. This is the function of ENSG00000141034 (GID4, GID complex subunit 4 homolog).