Active Transport of Membrane Components by Self-Organization of the Min Proteins.

Heterogeneous distribution of components in the biological membrane is critical in the process of cell polarization. However, little is known about the mechanisms that can generate and maintain the heterogeneous distribution of the membrane components. Here, we report that the propagating wave patterns of the bacterial Min proteins can impose steric pressure on the membrane, resulting in transport and directional accumulation of the component in the membrane. Therefore, the membrane component waves represent transport of the component in the membrane that is caused by the steric pressure gradient induced by the differential levels of binding and dissociation of the Min proteins in the propagating waves on the membrane surface. The diffusivity, majorly influenced by the membrane anchor of the component, and the repulsed ability, majorly influenced by the steric property of the membrane component, determine the differential spatial distribution of the membrane component. Thus, transportation of the membrane component by the Min proteins follows a simple physical principle, which resembles a linear peristaltic pumping process, to selectively segregate and maintain heterogeneous distribution of materials in the membrane. VIDEO ABSTRACT.

Low Water Potential and At14a-Like1 (AFL1) Effects on Endocytosis and Actin Filament Organization.

At14a-Like1 (AFL1) is a stress-induced protein of unknown function that promotes growth during low water potential stress and drought. Previous analysis indicated that AFL1 may have functions related to endocytosis and regulation of actin filament organization, processes for which the effects of low water potential are little known. We found that low water potential led to a decrease in endocytosis, as measured by uptake of the membrane-impermeable dye FM4-64. Ectopic expression of AFL1 reversed the decrease in FM4-64 uptake seen in wild type, while reduced AFL1 expression led to further inhibition of FM4-64 uptake. Increased AFL1 also made FM4-64 uptake less sensitive to the actin filament disruptor Latrunculin B (LatB). LatB decreased AFL1-Clathrin Light Chain colocalization, further indicating that effects of AFL1 on endocytosis may be related to actin filament organization or stability. Consistent with this hypothesis, ectopic AFL1 expression made actin filaments less sensitive to disruption by LatB or Cytochalasin D and led to increased actin filament skewness and decreased occupancy, indicative of more bundled actin filaments. This latter effect could be partially mimicked by the actin filament stabilizer Jasplakinolide (JASP). However, AFL1 did not substantially inhibit actin filament dynamics, indicating that AFL1 acts via a different mechanism than JASP-induced stabilization. AFL1 partially colocalized with actin filaments but not with microtubules, further indicating actin-filament-related function of AFL1. These data provide insight into endocytosis and actin filament responses to low water potential stress and demonstrate an involvement of AFL1 in these key cellular processes.