Jon Monroe monroejd at
Wed Sep 16 15:59:09 EST 1998

Magaly Rincon wrote:

>I am reading a review article regarding sorting between ER and Golgi and I
>came across with type I, II, III ER proteins.  Does anybody in the list
>know what they are?

I didn't know the answer but I happen to have just copied a review by
Galili et al. (Plant Molecular Biology, 1998, 38: 1-29), and I learned that
all refer to proteins directed to the ER but by different mechanisms. Type
I proteins contain cleavable, N-terminal signal sequences recognized by the
SRP (signal recognition particle).  Type II and Type III proteins do not
contain cleavable signal sequences and are not recognized by the SRP, but
do contain internal "signal anchor" sequences that help direct the protein
to the ER membrane and also serve as transmembrane domains.  In Type II
proteins the N-terminus ends up in the cytosol while the C-terminus ends up
in the ER lumen.  For Type III proteins the orientation is reversed.
Interestingly, membrane proteins that span the membrane multiple times can
be of any type.  The papers cited in this paragraph are: FEBS Lett 369:
76-79 '95, J Cell Bio 137: 555-562 '97, and PNAS 86: 5786-5790, '89.

Thanks for getting me to start reading this review!


  Jonathan Monroe
  Associate Professor
  Department of Biology
  James Madison University
  Harrisonburg, VA 22807
  voice:  540-568-6649 (office)
          540-568-6045 (lab)
  fax:    540-568-3333
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