A literature search yields that the size of sericin protein isolated from cocoon of silk worms ranges from 20-400 kDa in SDS-PAGE. The number of bands can be attributed to the reducing effect of SDS, but what is its molecular weight?
Uniprot suggests that there are three isoforms, one at ca 120 kDa and two around 70kDa. http://www.uniprot.org/uniprot/P07856
The number of bands can't be attributed to the reducing effect of SDS as SDS is not a reducing agent. A high number of bands or smear may be due to incomplete unfolding, or possibly differential glycosylation (if the protein is glycosylated). If isolated from cocoons, I would guess that some degradation may be occurring and you are seeing a range of protein species.
From Google Search
Molecular Weight
Extracting sericin using 1 per cent sodium deoxycholate solution followed by precipitation, using equal volume of 10 per cent trichloroacetic acid, shows molecular weight in the range of 17100 to 18460 (ref. 20).
Extraction of sericin by hot water shows molecular weight of 24000 by gel electrophoresis, whereas spray-drying method produced sericin of molecular weight 5000-50,000, with enzyme action 300-10,000 and 50,000 when it is extracted with aqueous urea at 100ºC .
the sericin should be extracted as the mix of differrent kDa. I have used in preserving e coli cells or mammalin cells. SO I can prvide you the sericin or reagents developed from it.
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