Figure 4.

The hypothetical evolutionary scenario leading to the Ssq1:Jac1 specialized partnership. The lines indicate functional protein–protein interactions, with thick solid lines indicating interactions of greatest strength and dotted lines indicating the weakest interactions. The stars represent amino-acid substitutions that affected biochemical properties of interacting proteins. The triangle indicates deletion in the J domain of Jac1. (A) Before gene duplication, Jac1 functioned with multifunctional mtHsp70. (B) A duplication of the mtHsp70 gene, resulting in two Ssc1 genes that initially functioned equally well with Jac1. (C) Mutations in the Ssc1 gene copy occurred, which became SSQ1 (D). A deletion in JAC1 occurred, reducing the size of the J domain and resulting in weak interaction with Ssc1. It is noted that this deletion event could have occurred either before—as indicated in (C)—or after other alterations in the gene that became SSQ1. The hypothesis that the changes in SSQ1 that facilitated interaction with deleted Jac1 occurred before, rather than after, the JAC1 deletion is appealing, because it could possibly explain the survival of cells immediately after the occurrence of the JAC1 deletion. (E) Over time, reciprocal changes in JAC1 and SSQ1 resulted in highly specific interactions. The colour change of Hsp70 from dark green (B) to light green (C) to blue (D,E) indicates the overall changes in amino acid that occurred after the SSC1 gene duplication. The Hsp70 encoded by the gene to become SSQ1 in (B) is indicated by ‘Ssq1'.