Abstract Type XII minicollagen chain association was studied using baculovirus-directed insect cell expression. Since insect cells contain low endogenous prolyl 4-hydroxylase activity, the mechanism of the effect of prolyl hydroxylation on trimer formation in this collagen could be studied directly by adding recombinant baculoviruses directing the synthesis of prolyl 4-hydroxylase. Prolyl 4-hydroxylase was shown to be involved in the trimeric assembly process of type XII collagen through its α subunit, and thus through its hydroxylase activity. The transmembrane protein type XIII collagen was also characterized by means of insect cell expression, for which purpose new antibodies against its non-collagenous domains NC2 and NC4 were generated, together with a pan-collagen antibody against collagenous sequences. Type XIII collagen α chains were found to form disulphide-bonded homotrimers, and this was enhanced by prolyl 4-hydroxylation. Analysis of the disulphide-bonding pattern of the eight cysteine residues of the α1(XIII) chains revealed that some of the cysteines in the NC1 domain, and possibly the cysteines at the junction of the COL1 and NC2 domains, are interchain-linked, while the cysteines in the NC4 domain are intrachain-linked. The three collagenous domains of type XIII collagen were shown to be in triple-helical conformation and have different thermal stabilities, i.e. 38±C for the COL1 domain, 49±C for COL2 and 40±C for COL3. Furthermore, it was shown that type XIII collagen is oriented in the plasma membrane of insect cells so that its non-collagenous N-terminus is intracellular and its mostly collagenous C-terminus is extracellular. Type XIII collagen was also found to be cleaved into the insect cell culture medium by a furin-like protease. The expression of various type XIII collagen deletion variants suggested that chain recognition and the association of type XIII collagen α chains into trimers occur in the N-terminal portion of this molecule. An internal in-frame deletion of residues 63-83 immediately adjacent to the transmembrane domain indicated that this short ectodomain sequence is necessary for the formation of disulphide-bonded trimers. Since a sequence homologous with these deleted residues was also found at the same plasmamembrane-adjacent location in other collagenous transmembrane proteins, this points to common features in their chain association.