[Genome] questions about PI distribution

[s58881139 at yahoo.com.tw]

Hi!
   
  May I ask one question about PI distribution ?
   
  As I know, typical PI distribution in eukaryocyte is a  typical trimodial distribution ( represent the peak in the near-neutral pI region).
   
  I read the note from UCSC proteome browser user's guide :
  "
  The mammalian pI distribution seems adequately modeled by an acidic Gaussian centered on pH 5.8, a pronounced trough at pH 7.4, and an alkaline Gaussian centered on pH 8.8. The near-neutral pI region has presumably been depleted by selective pressure -- cytoplasmic proteins are least soluble at their isoelectric point.  "
   
   
  Why I read a report the pI distribution in human mitochondrial proteome is a typical trimodial distribution ( represent the peak in the near-neutral pI region)?
   
  What is the selective pressure?
   
  Thank you for your help.
   
  Have a nice day.
   
                                                                   I-HUI

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