Anyone know a protein with real low pI ?

[Boris Steipe]

Deirdre O'Sullivan wrote:
> 
> I am looking for a protein of pI less than 3, which is NOT pepsin or an
> acidic protease. Other desirable qualities include fairly low molecular
> weight and it should be easily obtainable.

A search on Expasy <http://expasy.hcuge.ch/www/guess-prot.html> returns
only two proteins with a pI below 3 - and they don't exactly meet your
other criteria.

2 proteins found
        PSGL_ONCMY      (P12027)
                pI: 2.15, MW: 37141.05
                POLYSIALOGLYCOPROTEIN.
                
        VG40_BPML5      (Q05250)
                pI: 2.91, MW: 4443.52
                GENE 40 PROTEIN (GP40).

Actually with pKs of carboxyl groups being not lower than 3.9 and at
least one positive charge (on the N-terminus) this is not surprising.
677 proteins with a pK below 4 in Swissprot though. A number of
ferredoxins have pIs below 3.3.

Best wishes,
Boris

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