PMID: 27189939

Authors:

Hong Z, De Meulemeester L, Jacobi A, Pedersen JS, Morth JP, Andreasen PA, Jensen JK

Title:

Crystal Structure of a Two-Domain Fragment of Hepatocyte Growth Factor Activator Inhibitor-1: Functional Interactions between the Kunitz-Type Inhibitor Domain-1 and the Neighboring Polycystic Kidney Disease-like Domain.

Journal:

J Biol Chem. 2016 May 6. pii: jbc.M115.707240.

Abstract:

Hepatocyte growth factor activator inhibitor-1 (HAI-1) is a type-1 transmembrane protein and inhibitor of several serine proteases including hepatocyte growth factor activator and matriptase. The protein is essential for development as knock-out mice die in utero due to placental defects caused by misregulated extracellular proteolysis. HAI-1 contains two Kunitz-type inhibitor domains (Kunitz), which are generally thought of as a functionally self-contained protease inhibitor unit. This is not the case for HAI-1, where our results reveal how interdomain interactions have evolved to stimulate the inhibitory activity of an integrated Kunitz. Here we present an X-ray crystal structure of a HAI-1 fragment covering the internal domain and Kunitz-1. The structure not only reveals that the previously uncharacterized internal domain is a member of the polycystic kidney disease (PKD) domain family, but also how the two domains engage in interdomain interactions. Supported by solution small-angle X-ray scattering and a combination of site-directed mutagenesis and functional assays, we show that interdomain interactions not only stabilize the fold of the internal domain but also stimulate the inhibitory activity of Kunitz-1. By completing our structural characterization of the previously unknown N-terminal region of HAI-1, we provide new insight into the interplay between tertiary structure and the inhibitory activity of a multidomain protease inhibitor. We propose a previously unseen mechanism by which the association of an auxiliary domain stimulates the inhibitory activity of a Kunitz-type inhibitor i.e. the first structure of an intramolecular interaction between a Kunitz and another domain.