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Change in charge sign

The invariant residues discussed above clearly signal positions that are important to the common fold or function of the phosphatases. However, more subtle patterns of residue conservation can point to positions that are important to the specificity of different members of the protein family. For example, in the well known serine proteinases, the physico-chemical properties of the residue at the base of the S1 binding pocket contribute to the differing specificity of trypsin and chymotrypsin for peptide substrates. The residue is an Asp in trypsin-like enzymes which preferentially bind peptides at Lys or Arg, while this position is a Ser in chymotrypsin-like enzymes where the smaller side-chain allows binding of more bulky aromatic residues (e.g. see [44]). Within the protein phosphatase sequences shown in Figure 1, there are many positions at which amino-acids showing similar physico-chemical properties are conserved within one of the four sub-families, yet different residues are seen in other sub-families. The most striking positions are those where a charged residue is conserved within a sub-family of sequences (e.g. the PP1s and PP2As), yet is of different charge in other sub-families. Since charged residues are normally seen on the surface of a protein or performing important structural roles in the hydrophobic core, a change in conserved charge at a position suggests that the residue may be important for defining the functional specificity of the protein sub-family. Such positions are summarised in Table 3.

At position 4, a negative charge is predominant in all families except the PP5/PPT (Lys/Arg). At 142, Lys/Arg is conserved in the PP2B and PP2A sub-families, but Asp in the PP5/PPT group; PP1s conserve Ala/Ser at this position. At 151, Lys is conserved in the PP2Bs, but Glu in PP5/PPT, all other sequences have Tyr/Phe at this location. At 165 Glu/Asp is conserved in PP2Bs, but Lys/Arg/His predominate in PP2As and PP1s. At 213, His is conserved in PP2As, Asp/Glu in PP1s, whilst at 303 Arg is conserved in PP2Bs with Glu conserved in the PP1 family.

It is currently not possible to predict functions for these residues, however they may represent good targets for mutagenesis experiments aimed at probing the functional specificity of individual protein phosphatases.



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