Mutant/mutation
In the 'promoter swap mutant': the promoter of sub1 has been replaced with the asexual blood stage promoter ama1 (PBANKA_0915000). This promoter is active in asexual blood stages but not in gametocytes.

Protein (function)
The subtilisin-like protease SUB1, a bacterial-like serine protease, is a Plasmodium merozoite product essential for the parasite erythocytic cycle. In P. falciparum, SUB1 was shown to have a dual function. It is crucial for merozoite egress from erythrocytes and renders released merozoites competent for a new round of erythrocyte invasion.
The subtilisin-like protease SUB1 is expressed both in liver and blood schizonts, then secreted into the PV from vesicular organelles called exonemes just prior to schizont rupture. The proteolytic activity of SUB1 has been shown to be important for maturation and egress of invasive merozoites from both infected erythrocytes and hepatocytes.
During Plasmodium falciparum merozoite egress, PfSUB1 carries out the maturation of a family of papain-like proteins called the serine repeat antigens (SERAs), while for merozoite maturation the role of SUB1 is to undertake the processing of several merozoite surface proteins (MSPs).

Phenotype
SUB1 is expressed both in asexual blood stages and (male) gametocytes. Knock-out of the sub1 gene is not possible since it has an essential role in asexual blood stages. In the promoter swap mutant SUB1 is expressed in asexual blood stages but is down-regulated in (male) gametocytes.
Normal gametocyte production. Significant reduction in male gamete formation (exflagellation). The proportion of non-released male gametes (inside PVM/red blood cells) after activation was significantly higher in the SUB1/asex line compared to wt parasites. In addition, the proportion of non-released male gametes still possessing an intact PVM was significantly higher in the SUB1/asex line than in the wt parasites. Taken together, these results indicate a defect in PVM rupture of SUB1/asex male gametes and are consistent with the previously described role of SUB1 in schizont PVM rupture.

Additional information
From the paper:
'It was recently shown that PbSUB1 is also expressed in P. berghei sexual stages, but the function of the enzyme in gametocytes/gametes and its potential substrates in these stages are still unknown. Here we show that PbSUB1 is selectively expressed in male P. berghei gametocytes and that it localizes to male osmiophilic bodies (MOBs). Using conditional depletion of PbSUB1 expression or enzyme activity, we show that the protease plays a role in male gamete egress.
We also show for the first time that the PbSUB1 substrate PbSERA3 is expressed in gametocytes and processed by PbSUB1 upon gametocyte activation.'

'The SUB1/asex line was characterized by IFA and western blot analysis. In both cases, PbSUB1 was found to be expressed in transgenic schizonts in a similar manner to wt parasites, while it was not detected in SUB1/asex gametocytes'.



Other mutants