Reviewer #1 (Public Review):
This paper presents a detailed molecular investigation into the behavior of PIP5K kinase, a membrane associated enzyme that catalyzes the production of PIP2 from PIP in cell membranes. Building on previous work on this system, the researchers use single-molecule fluorescence microscopy to study how the oligomeric state of PIP5K impacts membrane binding, PIP phosphorylation, and compositional patterning of lipid domains leading to stochastic bistability in membranes.
A highlight of this study is the extensive experimental approaches that combine various single-molecule analyses, including diffusion and residence time distributions, as well as macroscopic measurements of membrane binding isotherms and PIP2 production. With this, it becomes evident that PIP5K exists in both monomeric and higher-order oligomeric forms, with the latter potentiating catalytic activity. This coupled to cooperative binding to the membrane linked to PIP2 production leads to a positive feedback system where patterning of the lipid composition emerges with stochastic bistable behavior, with oligomerization of the kinase acting as a modulating factor. This aspect of the research is interesting as it connects the higher-order oligomerization of the protein kinase to a means of modulating self-organization of the lipids within the cell membrane, a phenomenon that may be important for optimizing cellular signalling in biology.
The majority of the studies are carried out carefully and with exquisite single-molecule approaches. However, a weakness of the study is that the ultimate conclusion of the activity linked specifically to dimerization is not clearly supported by the data. The results presented reflect a comparison of monomers vs. oligomers, without a clear identification of conditions where dimers persist. The mutation constructed to disrupt dimerization shifts the system to monomers with an associated decrease in catalytic activity. However, this finding does not provide a strong connection to the dimer state, but rather the loss of the effect when oligomerization is disrupted. Other properties of the protein may be impacted, such as stability and fold, as well as the overall binding propensity to the membrane. The catalytic activity measured per PIP5K molecule does indicate that an increased density for the wild-type protein leads to an increase in the rate of PIP2 production, providing evidence that oligomerization increases function. Yet, many of the results throughout the paper provide support for general oligomerization rather than dimerization, and so further investigation is needed in order to clarify the interpretation in what is otherwise an interesting system and study.



