Structural Analysis of Major Species Barriers Between Humans and Palm Civets for SARS Coronavirus Infections

University of Minnesota

^* To whom correspondence should be addressed. Email: lifang{at}umn.edu.

	Abstract

It is believed that a novel coronavirus, SARS-CoV, was passed from palm civets to humans and caused the epidemic of Severe Acute Respiratory Syndrome (SARS) in 2002-2003. The major species barriers between humans and civets for SARS-CoV infections are the specific interactions between a defined receptor-binding domain (RBD) on a viral spike protein and its host receptor, angiotensin-converting enzyme 2 (ACE2). This study has constructed a chimeric ACE2 bearing the critical N-terminal helix from civet and the remaining peptidase domain from human, and showed that it has the same receptor activity as civet ACE2. This study has further determined crystal structures of the chimeric ACE2 complexed with RBD from various human and civet SARS-CoV strains. These structures, combined with a previously determined structure of human ACE2 complexed with RBD from a human SARS-CoV strain, have revealed structural basis for understanding the major species barriers between humans and civets for SARS-CoV infections. They show that the major species barriers are determined by interactions between four ACE2 residues (31, 35, 38, and 353) and two RBD residues (479 and 487), that early civet SARS-CoV isolates were prevented from infecting human cells due to imbalanced salt bridges at the hydrophobic virus/receptor interface, and that SARS-CoV has evolved to gain sustained infectivity for human cells by eliminating unfavorable free charges at the interface through stepwise mutations at 479 and 487 positions. These results enhance our understanding of host adaptations and cross-species infections of SARS-CoV and other emerging animal viruses.