Widespread dual targeting of proteins in land plants: when, where, how and why

Abstract

Since the discovery of the first dual targeted protein in plants in 1995 the number of dual targeted proteins in plants has grown to over 250 proteins. Much work and investigations have focused on identifying how or what makes a protein dual targeted. Recently, more research has focused on the evolution and conservation of dual targeting of proteins in plants. This new work has demonstrated that dual targeting arose early within the evolution of plants and because it is rarely lost, once gained, it must be under some positive selection pressure. The possible reasons as why proteins are dual targeted and why it was conserved during the evolution of plants are discussed.

Figure 1. Mechanisms of dual targeting. Dual targeting can be achieved by the use of two translation products or a single translation product. The use of two translation products is usually controlled at the mRNA level. This can either be the result of alternative transcription initiation or differential splicing resulting in two separate mRNAs encoding different targeting signals. Alternative translation initiation can also be utilized to produce two different proteins with different targeting signals. Single translation products can be dual targeted in two ways. The first uses two separate targeting signals on different ends of the protein and the second uses an ambiguous targeting signal which directs the protein to two organelles.