Family of Kunitz proteins from trophoblast: expression of the trophoblast Kunitz domain proteins (TKDP) in cattle and sheep.

Here we report the molecular cloning of several members of a family of novel proteins expressed by the ruminant trophoblast, known as the trophoblast Kunitz domain proteins (TKDPs). Each contains a carboxyl-terminal module of approximately 64 amino acids belonging to the Kunitz family of serine proteinase inhibitors. These Kunitz modules are preceded by one or more structurally related domains, each about 80 amino acids long. The function of these domains is unclear. The TKDPs differ considerably in sequence identity, with much of the diversity due to variability in the amino-terminal domains. However, nine of the ten Kunitz domains described here are themselves unique, ranging in amino acid sequence identity from 90% to 53% to each other and averaging only about 50% identity with bovine pancreatic trypsin inhibitor (BPTI). The "warhead" P1 residues, which govern specificity, are themselves variable and include some unusual amino acids, such as Asn, Thr, and Ile, as well as the more common Lys. The Kunitz domains of three of the TKDPs lack the conserved cysteines at positions 14 and 38 (BPTI numbering) that normally contribute to the orientation of the inhibitory loop. Northern blotting and reverse transcription-polymerase chain reaction (RT-PCR) demonstrated that the TKDP genes do not exhibit identical expression patterns during trophoblast development, although mostly are expressed maximally during early pregnancy. It is possible that the TKDPs provide a broad range of specificities against maternal proteinases that might be damaging to the trophoblast during pregnancy.