Protein precursor molecule, post-translational

Protease is responsible for cleaving these precursor molecules to produce the final structural proteins of the mature virion core. By preventing post-translational cleavage of the Gag-Pol polyprotein, protease inhibitors (Pis) prevent the processing of viral proteins into functional conformations, resulting in the production of immature, noninfectious viral particles (Figure 49-4). Protease inhibitors are active against both HIV-1 and HIV-2 unlike the NRTTs, however, they do not need intracellular activation. 

In prokaryotes, RNA transcribed from protein-coding genes (messenger RNA, mRNA), requires little or no modification prior to translation. In fact, many mRNA molecules begin to be translated even before RNA synthesis has finished. However, ribosomal RNA (rRNA) and transfer RNA (tRNA) are synthesized as precursor molecules that do require post-transcriptional processing (see Topics G9 and G10, respectively). 

Beyond translation, proteins and peptides may be further modified by metabolic events within the cell. For instance, the hydroxyproline and hydroxylysine in collagen are formed from proltne and lysine that are hydroxyl-ated while they are part of the precursor protein molecule. A protein may also be post-translationally modified by cleavage of select bonds and/or by addition or subtraction of various kinds of groups. Insulin, for instance, is formed cleavage of a larger protnsultn molecule. Similar events occur in the activation of zymogens. 

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