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Protein Instability Index Calculator
Dipeptide composition in a protein's primary structure is predicted to play a role in protein stability. PEST hypothesis and Protein Instability Index (PII) are two proposals relating the protein stabilty to the primary strucrure (1, 2). The mechanism of degradation in relation to the primary sequence is not understood well. PII is a statistical analytical procedure based on Dipeptide Instability Weight Value (DIWV)(2).
While the above mentioned hypotheses considers only the in vivo protein stabilty, one of  the  proteins  we cloned overespressed  in our lab was caluculated to have a PII of 40.10, a value just outside the stable region (PII of 1 - 40 are considered relatively stable). As predicted our expressed protein degraded rapidly (in days) upon storage at 4 ºC. 
 
We wrote a computer program based on Guruprasad et al.'s work, in order to help ourselves in calculating PII for our experiments. We were able to calculate the exact predited PII value for our protein.
We are making the program available here for anyone to use. We are not responsible for any errors in the program. We appreciate if you could share your results and inform us if you find any problem with the calculator to improve it. 

References:

Guruprasad, K., B. V. Reddy, et al. (1990). "Correlation between stability of a protein and its dipeptide composition: a novel approach for predicting in vivo stability of a protein from its primary sequence." Protein Eng 4(2): 155-61.