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Porins are found in the outer membranes of Gram-negative bacteria,
mitochondria and chloroplasts, where they form ion-selective channels for
small hydrophilic molecules (up to ~600 Da) [1,2]. X-ray structure analyses
of several bacterial porins [3-5] have revealed a 16-stranded anti-parallel
beta-barrel structure enclosing the transmembrane pore, by contrast with
all other integral membrane proteins described to date, which are alpha-
helical. Three subunits form a trimer; the 3-fold axis is approximately
parallel to the barrel axes and is assumed to be perpendicular to the
membrane plane .
From the range of porins now known, similarities have been observed between
porins from different species, and between porins of different specificity
within the same species. But most porins cannot be related to each other on
the basis of sequence alone, and this is reflected in the lengths of the
known porin sequences, which range from 282-483 residues/monomer.
EUKARYTPORIN is a 4-element fingerprint that provides a signature for
eukaryotic porins. The fingerprint was derived from an initial alignment
of 3 sequences: motif 2 includes the well-conserved region encoded by
PROSITE pattern EUKARYOTIC_PORIN (PS00558). Two iterations on OWL19.1 were
required to reach convergence, at which point a true set comprising 4
sequences was identified. The fingerprint is selective for this family
alone, and there is no cross-reaction with E.coli/Salmonella-type porins,
or with porins from Neisseria.
An update on SPTR37_9f identified a true set of 13 sequences.