Along with Cell
My notes on the web
Friday, November 9, 2012
Why is the observed Western Blot band size different from predicted size?
The predicted M.W. is based on protein sequence analysis; however, some factors might lead to an observed band size that is different from the predicted size. The reasons might include:
1.Post-translational modification (PTM):
a. Some post-translational modifications might lead to increased protein size, including
phosphorylation, acetylation, methylation, glycosylation, sumoylation, ubiquitination,
etc.
b. Some post-translational modifications might lead to decreased protein size including
phosphatidylethanolamine conjunction (e.g. LC3-II)
c. Some proteins may be cleaved to form an active or mature form; this process will
lead to a decreased protein size (e.g. Notch activation, Caspase activation, etc.)
d. Some websites provide useful PTM information
i. HPRD http://www.hprd.org/
ii. ProSite http://www.expasy.org/prosite/
iii. ELM http://elm.eu.org/
iv. CBS data sets http://www.cbs.dtu.dk/databases/
v. CBS prediction Servers http://www.cbs.dtu.dk/services/
2.mRNA splice variants (Isoforms):
Through alternative splicing, one gene can generate different proteins with different M.W. Regulation of alternative splicing depends upon cell type, conditions, etc.
3.Multimerization:
Some proteins could form dimers or multimers, increasing the M.W. This phenomenon usually can be found in reducing gel condition; however, strong interactions may still be seen with higher molecular weight proteins even in denaturing gel.
4.Protein charge:
The observed size could also potentially be influenced by the protein charge
5.Different species:
Different species likely have different protein sequence and PTM, which can lead to a different protein M.W.
Source: http://www.genetex.com/WebPage/Product/PredictedTarget.aspx
Wednesday, February 8, 2012
Trypan Blue
Trypan blue is a vital stain used to selectively colour dead tissues or cells blue. It is a diazo dye. Live cells or tissues with intact cell membranes are not coloured. Since cells are very selective in the compounds that pass through the membrane, in a viable cell trypan blue is not absorbed; however, it traverses the membrane in a dead cell. Hence, dead cells are shown as a distinctive blue colour under a microscope. Since live cells are excluded from staining, this staining method is also described as a dye exclusion method. (from:Wikipedia)
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