Re: How is cellular RNA protected from endogenous RNases

Here is what is thought to be important for the most eukaryotic cellular 
mRNAs' protection from RNase activity. Basically, cellular mRNAs interact 
with some proteins in order to be protected from RNAses. Eventually, all 
mRNAs are degraded, but depending on the need that particular cell have 
some mRNAs live longer than other. You can find more information 
in “Spångberg K, Wiklund L Schwartz S (2001). Binding of the La 
autoantigen to the hepatitis C virus 3' untranslated region protects the 
RNA from rapid degradation in vitro Journal of General Virology, 82, 113-
120”, and related references (see below). Here are the essentials from 
the manuscript: "Most eukaryotic cellular mRNAs contain a Cap structure 
at the 5' end and a poly(A) tail at the 3' end. These structures have 
multiple functions and affect splicing, transport, translation and 
stability of mRNAs. For example, the Cap and the poly(A) tail act 
synergistically to promote translation of cellular mRNAs (Sachs, 1997 ; 
Wickens et al., 1997 ).... The Cap and the poly(A) tail interact with 
cellular proteins that protect the mRNAs from exonucleolytic degradation 
(Sachs, 1997 ; Wickens et al., 1997 ). Decapping and deadenylation 
precede mRNA degradation (Ross, 1995). Thus, the presence of the Cap and 
the poly(A) tail on the mRNAs results in protection of the mRNAs from 
RNases and prevents untimely degradation of the cellular mRNAs. For 
example the poly(A) binding protein binds to the poly(A) tail and 
inhibits premature mRNA degradation. Interestingly, the poly(A) binding 
protein stabilizes mRNAs in the absence of a poly(A) tail, if tethered to 
the mRNA (Coller et al., 1998 ), demonstrating that it is the poly(A) 
binding protein and not the poly(A) tail itself that protects the mRNA 
from degradation." 
Wickens, M., Anderson, P. & Jackson, R. J.(1997). Life and death in the 
cytoplasm: messages from the 3' end. Current Opinion in Genetics & 
Development 7, 220-232.  Sachs, A. B.(1997). Starting at the beginning 
middle and end: translation inititation in eukaryotes. Cell 89, 831-838. 
Ross, J.(1995). mRNA stability in mammalian cells. Microbiological 
Reviews 59, 15-95.
Coller, J. M., Gray, N. K. & Wickens, M. P.(1998). mRNA stabilization by 
poly(A) binding protein is independent of poly(A) and requires 
translation. Genes & Development 12, 3226-3235.