another GST story #2

[Terry Hanzlik]

On 15 Jul 1994 15:14:16 GMT, Stephen R. Lasky wrote:

(some stuff deleted)

>>In general though,  we use denaturation in the presence of SDS so that a
>>protein will migrate at a rate relative to its mass and not be effected by
>>amino acid composition.

>That's the generally accepted therory and, no doubt, many if not most 
>proteins fit nicely into that scheme. But there are exceptions, just one 
>example (might be interesting for Jean-Marc and Emma): There is a protein 
>called dTAF40 because on a SDS gel it has the apparent molecular mass of 40 
>kd. Molecular cloning of the cDNA revealed a molecular mass of only 29 kd 
>(Cell 75, 519-530, 1993). Could anybody kindly give an explanation for this 
>unusual migration on a SDS gel? 

>Matthias Zeiner
>Inst. Biol. Chem.
>Heidelberg

Dear Matthias,

It is highly likely that the proteins in your question
that migrate as larger proteins on SDS-PAGE have a high proline
content.  This phenomenon involving proline has been noted many 
times before; for
example see Pham and Sivasubarmanian in GENE 122, 345 (1993) or
Ziemer, Mason and Carlson in JBC 257, 11176 (1982).  My own
personal experience with such a protein is one from an insect 
small RNA virus which is 17 kDa yet migrates at Mr = 24k (>140% 
more apparent MW).  
It is a PEST protein (a property correlated with rapid turnover
and possessed by many regulatory genes and oncogenes) and has a 
49% of P, E, S, and T.  
However, to the nub of your question as to why proline does this,
I and the literature I've seen are still unaware.  Anybody else
have a handle on this?

Terry Hanzlik, terryh at ento.csiro.au
CSIRO Division of Entomology
Box 1700
Canberra, ACT  2601
Australia