It depends what kind of protein u wish to study and how much did you process it. If for example you didn't process the tissue at all, and you wish to study sensitive proteins like cytokines, you better keep it in -80C at least (or liquid nitrogen). If you already processed the sample (lysis etc...) and denaturated the proteins, you can probably do with -20C.

We usually keep it -80C all the time. After denaturation you can kkep it in either -80C or -20C.

Got it. Thank you.