  Prion diseases {\it (e.g. Creutzfeldt-Jakob disease (CJD), variant CJD
(vCJD), Gerstmann-Str$\ddot{\text{a}}$ussler-Scheinker syndrome (GSS), Fatal
Familial Insomnia (FFI) and Kuru in humans, scrapie in sheep, bovine spongiform
encephalopathy (BSE or `mad-cow' disease) and chronic wasting disease (CWD) in
cattles)} are invariably fatal and highly infectious neurodegenerative diseases
affecting humans and animals. However, by now there have not been some
effective therapeutic approaches or medications to treat all these prion
diseases. Rabbits, dogs, and horses are the only mammalian species reported to
be resistant to infection from prion diseases isolated from other species.
Recently, the $\beta$2-$\alpha$2 loop has been reported to contribute to their
protein structural stabilities. The author has found that rabbit prion protein
has a strong salt bridge ASP177-ARG163 (like a taut bow string) keeping this
loop linked. This paper confirms that this salt bridge also contributes to the
structural stability of horse prion protein. Thus, the region of
$\beta$2-$\alpha$2 loop might be a potential drug target region. Besides this
very important salt bridge, other four important salt bridges
GLU196-ARG156-HIS187, ARG156-ASP202 and GLU211-HIS177 are also found to greatly
contribute to the structural stability of horse prion protein. Rich databases
of salt bridges, hydrogen bonds and hydrophobic contacts for horse prion
protein can be found in this paper.
