ABSTRACT
Phosphoinositide signalling systems exist in all eukaryotes. A high degree of evolutionary conservation is found at the functional level, but distinct phylogenetic differences are also becoming evident. Although the nuclear phosphoinositide system is likely to be a primordial forerunner of the plasma membrane system, relatively little is known about it. However, nuclear phosphoinositides might have far more diverse roles than hitherto envisaged and interact specifically with regulatory proteins containing phosphoinositide-binding domains. A novel family of proteins, so far only identified in plants, display domain structures that might link phosphoinositide metabolism to nuclear function in an unexpected way.
Subject(s)
Cell Nucleus/metabolism , Phosphatidylinositols/metabolism , Amino Acid Sequence , Animals , Binding Sites/genetics , Molecular Sequence Data , Nuclear Proteins/genetics , Nuclear Proteins/metabolism , Phosphatidylinositol 4,5-Diphosphate , Phosphatidylinositol Phosphates/metabolism , Plant Cells , Protein Binding , Sequence Homology, Amino Acid , Signal TransductionABSTRACT
A full-length cDNA encoding a novel protein named PARF-1 was isolated from Arabidopsis thaliana. PARF-1 is the first eukaryotic protein to be identified that displays a domain structure which includes a FYVE-finger domain, a Pleckstrin Homology (PH) domain, as well as multiple Regulator of Chromosome Condensation-1 (RCC1) repeats. Northern blot analysis revealed that PARF-1 mRNA is present at high levels in flowers, but only at very low levels in other tissues. Recombinant PARF-1 fusion proteins expressed in E. coli were found to display unique binding specificities for monophosphorylated phosphoinositide lipids. The unusual domain structure of PARF-1 combined with its phosphoinositide specificity suggests that it may fulfil unexpected functions in higher plants.
Subject(s)
Arabidopsis Proteins/genetics , Arabidopsis/genetics , Phosphatidylinositols/metabolism , Amino Acid Sequence , Arabidopsis Proteins/metabolism , Binding Sites/genetics , Binding, Competitive , Blotting, Northern , Gene Expression Regulation, Plant , Molecular Sequence Data , Plant Structures/genetics , RNA, Messenger/genetics , RNA, Messenger/metabolism , Sequence Homology, Amino Acid , Zinc Fingers/geneticsABSTRACT
A cDNA clone encoding a phosphatidylinositol 3-kinase (PtdIns 3-kinase) has been characterized from Medicago truncatula, Mtpi3k, that is highly homologous to their counterparts from soybean (over 84%). The results suggest the presence of at least two genes coding PtdIns 3-kinases in M. truncatula. Mtpi3k transcript levels increased in nodules, compared with non-infected roots. Strikingly, Mtpi3k mRNA accumulated in young elongating stems at higher levels than that observed in other organs. Enhanced transcription of genes coding PtdIns 3-kinases might occur in tissues experiencing a high degree of vesicle trafficking and cell elongation.
Subject(s)
Gene Expression Regulation, Enzymologic , Gene Expression Regulation, Plant , Medicago/enzymology , Medicago/genetics , Phosphatidylinositol 3-Kinases/genetics , Meristem/enzymology , Plant Leaves/enzymology , Plant Roots/enzymology , Plant Shoots/enzymology , Plant Stems/enzymology , RNA, Messenger/genetics , Restriction MappingABSTRACT
Pollen tube growth is dependent on a dynamic actin cytoskeleton, suggesting that actin-regulating proteins are involved. We have examined the regulation of the lily pollen-specific actin-depolymerizing factor (ADF) LlADF1. Its actin binding and depolymerizing activity is pH sensitive, inhibited by certain phosphoinositides, but not controlled by phosphorylation. Compared with its F-actin binding properties, its low activity in depolymerization assays has been used to explain why pollen ADF decorates F-actin in pollen grains. This low activity is incompatible with a role in increasing actin dynamics necessary to promote pollen tube growth. We have identified a plant homolog of actin-interacting protein, AIP1, which enhances the depolymerization of F-actin in the presence of LlADF1 by approximately 60%. Both pollen ADF and pollen AIP1 bind F-actin in pollen grains but are mainly cytoplasmic in pollen tubes. Our results suggest that together these proteins remodel actin filaments as pollen grains enter and exit dormancy.