Cytosolic proteins from tobacco pollen tubes that crosslink microtubules and actin filaments in vitro are metabolic enzymes

In plant cells, many processes require cooperative action of both microtubules and actin filaments, but proteins mediating interactions between these cytoskeletal members are mostly undiscovered. Here, we attempt to identify such proteins by affinity purification. Cytosol from Nicotiana tabacum (tobacco) pollen tubes was incubated first with actin filaments, and then proteins eluted from the actin were incubated with microtubules, and finally those microtubule-binding proteins were pooled in an active fraction. This fraction bundled actin filaments but not microtubules. However, when the fraction was added to both actin and microtubules, large bundles resulted, containing both polymers, regardless of the order of addition of components. Similar results were obtained when the order of affinity purification was reversed. The four most abundant bands from the fractions were identified from peptide fragments analyzed by mass spectrometry. The same four proteins were identified regardless of the order of affinity purification. The proteins are: homocysteine methyltransferase, phosphofructokinase, pyruvate decarboxylase, and glucan protein synthase (reversibly glycosylated protein). These results suggest the importance of structuring metabolism within the confines of the pollen tube cytoplasm.