Are plant formins integral membrane proteins?

Abstract

formin homologs. genome sequence and analyzed their predicted protein sequences. Surprisingly, some of them lack parts of the conserved formin-homology 2 (FH2) domain and the majority of them seem to have signal sequences and putative transmembrane segments that are not found in yeast or animals formins. formins of sequence motifs typical or transmembrane proteins suggests a mechanism of membrane attachment that may be specific to plant formins, and indicates an unexpected evolutionary flexibility of the conserved formin domain.

Background:

formin homologs.

Results:

genome sequence and analyzed their predicted protein sequences. Surprisingly, some of them lack parts of the conserved formin-homology 2 (FH2) domain and the majority of them seem to have signal sequences and putative transmembrane segments that are not found in yeast or animals formins.

Conclusions:

formins of sequence motifs typical or transmembrane proteins suggests a mechanism of membrane attachment that may be specific to plant formins, and indicates an unexpected evolutionary flexibility of the conserved formin domain.

Background

]. For many morphogenetic mechanisms, the question of evolutionary conservation remains unresolved because available information is limited to one or a few model organisms. For example, this is the case for the molecular mechanisms that ensure the communication between the cytoskeleton and the surface of the cell. However, the recent increase in the data available from a number of genome projects allows wide-ranging searches for homologs of known components of signaling and morphogenetic pathways. The results of such searches can lead both to experimentally testable hypotheses and to general conclusions regarding the evolution of morphogenetic processes.

].

]) by recent eukaryotes.

Results and discussion

. These putative genes and their predicted protein products will be referred to henceforth as AtFORMINs 1 to 8.

Chr, chromosome number.

], failure to identify exons correctly may explain the apparent deletion of this region of the FH2 domain. The possibly mispredicted intron encoding subdomain g of AtFORMIN4 is split by a frameshift mutation, however. Although this could reflect a sequencing error, the possibility remains that plant formin homologs have a modular structure within the FH2 domain at the gene level, and that at least some of the FH2-related sequences within predicted introns are vestiges of exons lost by mutation.

and Materials and methods.

. Only the 'highly likely' membrane-spanning segments are shown.

).

] are highlighted in green, novel branches in yellow.

]. Interestingly, the FH1 domains of AtFORMINs 2, 7 and 8 are extremely rich in serine (up to 20%) and contain stretches of up to seven consecutive serine residues.

formins. Aliphatic (I, L, V), aromatic (F, H, W, Y) and other potentially hydrophobic (A, C, G, K, M, R, T) amino acids are highlighted

], we can at present neither prove nor exclude the possibility that plant formins contribute to similar functional modules to their animal and fungal counterparts. The question of whether these proteins have a direct role in cytokinesis, in mitotic spindle localization, or in some other cellular process, possibly involving cytoskeleton rearrangement or cell-surface growth, will have to be answered experimentally.

Conclusions

formins appear to be integral membrane proteins. This indicates a mechanism of membrane localization that may be specific to plants and functionally related to a possible role for formins in the communication between the plant cell and extracellular structures.

formin homologs and protein sequence prediction

, respectively), verifying the statistical significance of the initial PatMatch results.

]. Only the longest predicted ORFs were subjected to further analysis.

Sequence alignment and domain structure analysis

].

]). Results of both methods were in agreement, with the exception of AtFORMIN5, which was predicted to be membrane-anchored by NN but cytoplasmic by HMM.

Construction of the evolutionary tree

] by NEIGHBOR. The consensus tree was determined by CONSENSE and plotted using DRAWTREE.

Acknowledgements

This work has been supported by the Grant Agency of the Czech Republic Grant 204/98/0482 and by the Czech Ministry of Education Program J13/98:113100003. I thank J. Flegr for helpful discussion.