The discovery and rapid exploitation of the Pichia platform demonstrates that this type of yeast is a promising alternative to the mammalian culture approach. Unfortunately, its protein glycosylation pattern differs from mammalian/human cells. Both bioindustrial and academic scientists are working hard to circumvent this drawback.
The advantages of Pichia are the ability to grow to high cell density and the presence of an efficient growth-to-protein production switch. For this organism, optimization of its protein production machinery is less developed.
The process of secreted recombinant protein production by both mammalian and Pichia hosts follows the same straightforward pathway—transcription, translation, and post-translational modification. ProteoNic contributes to yield improvement by focusing on the translation step.
The translation process can be divided into three sub-processes: initiation, elongation, and termination. The initiation phase is the rate-limiting step. It starts when a variety of proteins binds to the mRNA and to parts of the translation machinery (the ribosome subunits). Upon completing the proper binding sequence, translation begins, and the fully functional mRNA-ribosome complex will start to build the peptide chain.
Elongation of the peptide occurs by the translocation of the ribosome step-by-step along the mRNA. At each point new amino acids bind to the nascent protein. Termination follows as the code in the mRNA orders the ribosome to stop adding more amino acids. The protein is then released, and the protein synthesizing machinery is carefully disassembled and recycled.
The freshly produced proteins often need further processing to become fully functional and/or transported. These steps will not be considered in this review.
Few genetic technologies have been reported to increase the translation yield or the translatability of mRNA. The only commonly used technique is codon optimization in which the genetic code of a gene of interest is adapted to the presence of the tRNAs that are most used by the cell.
mRNA expression per se was found to be a poor predictor of protein levels as a low-copy, efficiently translated mRNA molecule can easily out-compete a high-copy, poorly translated mRNA. This is exemplified by the fact that the ratio of protein molecules to mRNA molecules in a cell depends on the protein concerned and this ratio varies by at least three orders of magnitude. The beginning (the 5´ end) of the mRNA is important for regulating translation because its structure affects ribosome recruitment and scanning.