Abstract The nucleocapsid (N) protein of SARS coronavirus
(SARS_CoV) is a major structural component of virions, which
appears to be a multifunctional protein involved in viral RNA
replication and translation. Heterogeneous nuclear ribonucleoprotein
A1 (hnRNP A1) is related to the pre-mRNA splicing
in the nucleus and translation regulation in the cytoplasm. In this
report, based on the relevant biophysical and biochemical assays,
the nucleocapsid protein of SARS_CoV (SARS_N) was discovered
to exhibit high binding affinity to human hnRNP A1. GST
pull-down results clearly demonstrated that SARS_N protein
could directly and specifically bind to human hnRNP A1 in vitro.
Yeast two-hybrid assays further indicated in vivo that such binding
relates to the fragment (aa 161C210) of SARS_N and the
Gly-rich domain (aa 203C320) of hnRNP A1. Moreover, kinetic
analyses by surface plasmon resonance (SPR) technology revealed
that SARS_N protein has a specific binding affinity
against human hnRNP A1 with KD at 0.35 0.02 lM (kon =
5.83 0.42 103 M1 s1 and koff = 2.06 0.12 103 s1). It
is suggested that both SARS_N and hnRNP A1 proteins are possibly
within the SARS_CoV replication/transcription complex
and SARS_N/human hnRNP A1 interaction might function in
the regulation of SARS_CoV RNA synthesis. In addition, the
determined results showed that SARS_N protein has only one
binding domain for interacting with human hnRNP A1, which
is different from the mouse hepatitis virus (MHV) binding case
where the nucleocapsid protein of MHV (MHV_N) was found
to have two binding domains involved in the MHV_N/hnRNP
A1 interaction, thereby suggesting that SARS_N protein might
carry out a different binding mode to bind to human hnRNP
A1 for its further function performance in comparison with