Signal anchors are "uncleaved signal peptides" which has no SPase recognition
site, thus is not cleaved. Introduction of the HMM model in SignalP version 2,
made it possible to distringuish signal peptides from signal anchors.
Signal anchors are type II membrane proteins and can be found in SWISS-PROT
by searching for the flowing in the feature table (FT) field,
"SIGNAL-ANCHOR (TYPE-II MEMBRANE PROTEIN)".
Other signals for secretion
Many different signal motifs are found and used for different targeting to
various subcellular localizations. SignalP was not developed to predict these
diverse targeting signals, but is mentioned here as background knowledge.
TAT (Twin-arginine translocation)
Twin arginine signal peptides contain a conserved twin argine motif (RR)
within the signal peptide. These signal peptides are in general longer and less hydrophobic
than the classical signal peptides predicted by SignalP. Often, SignalP, is
capable of correctly classifying these proteins as secretory, although the position of the
predicted cleavage site is wrong. More information about Tat signal peptides can be found at
Interpro using the following accession number IPR006311.
Prokaryotic lipoprotein cleavage sites are not predicted using SignalP. Prokaryotic lipoproteins are cleaved by a specific lipoprotein
signal peptidase, Lsp or signal peptidase II. This peptidase recognizes a
conserved sequence and cuts upstream of a cysteine residue to which a glyceride-fatty acid
lipid is attached. The cleavage sites of these proteins differ considerably from those
cleaved by the standard prokaryotic signal peptidase (SpaseII).
about prokaryotic lipoproteins and their consensus sequence can be found in the PROSITE entry PROKAR_LIPOPROTEIN.
Information can also be found at Interpro IPR000437.
For prediction of prokaryotic lipoproteins we recommend using the
Non-classical and leaderless secreted proteins
Not all secretory proteins carry signal peptides
or similar targeting signals. Some proteins enter the non-classical secretory pathway
without any currently known sequence motif. These proteins are mostly growth factors
and extracellular matrix binding proteins. For prediction of such proteins we
recommend the SecretomeP