The RNases are made by a part of the S-locus, a huge, intractable stretch of DNA. Although the female-specific product of the S-locus has long been known to be the S-RNase, the male-specific product (made by the pollen tube) has eluded scientists for a decade. It is now identified as a regulator of ubiquitination that seems to sentence all but self S-RNases to degradation.
Through a brute force sequencing approach, Paja Sijacic, Teh-hui Kao (Penn State University), and colleagues found that the petunia pollen S-component is the SLF F-box protein. Normally, a haploid pollen grain expresses only one S-allele. But forced expression of two different S-alleles alters pollen rejection. The group now shows that two different SLFs can likewise alter pollen rejection, thus confirming that SLF is the male incompatibility protein.
Hong Qiao, Yongbiao Xue, and colleagues (Chinese Academy of Sciences, Beijing, China) found that SLFs from snapdragon bind to both self and non-self S-RNases. But only the non-self enzymes were ubiquitinated and thus degraded.
How SLF prevents degradation of its own S-RNase is not clear. Kao guesses they may have matching interaction domains that either block the ubiquitination site or alter the F-box so that it cannot interact with other SCF components. ▪