Phytoglobin: a novel nomenclature for plant globins accepted by the globin community at the 2014 XVIII conference on Oxygen-Binding and Sensing Proteins

Hemoglobin (Hb) is a heme-containing protein found in the red blood cells of vertebrates¹. Hemoglobin-like proteins are also found in other tissues of vertebrates where they are given tissue-specific names that help to identify their locations and distinguish them from red blood cell Hb^2,3. For many years, the only known Hb-like molecule in plants was leghemoglobin (Lb), a protein induced as a result of the symbiotic relationship between legume plants and nitrogen-fixing bacteria⁴. The discovery that other Hb-like proteins existed in plants not capable of symbiotic relationships led to the term “nonsymbiotic Hbs (nsHbs)” to differentiate them from the Lbs⁵. While this terminology was adequate in the early stages of research on the protein, the complexity of the research in this area necessitates a change in the definition of these proteins to delineate them from red blood cell Hb, in keeping with the terminology for other Hb-like proteins, such as myoglobin in muscle, neuroglobin in neuron tissue and cytoglobin in vertebrate cell cytoplasm^2,3. In 2001 Hunt et al.⁶ classified plant Hbs as globin (GLB)0, GLB1, GLB2, GLb3 and GLBS corresponding to undetermined (mostly liverwort and moss) nsHbs, angiosperm nsHbs class/type 1 and nsHbs class/type 2, truncated Hbs and symbiotic Hbs (which included Lbs), respectively. However, an epithet for plant Hbs was absent in this nomenclature and distinctive characteristics for each category were not fully defined resulting in an incomplete classification system.

At the 2014 XVIII Conference on Oxygen-Binding and Sensing Proteins, the group devoted to the study of heme-containing proteins, the above issue was discussed and a consensus was reached on a proposed name change. Phytoglobin (phyto, plant; globin, heme-containing protein folding structurally similar to the sperm whale myoglobin structure whose heme-Fe is invariably coordinated at the proximal site by His F8), abbreviated as Phytogb, was proposed as a logical, descriptive name to describe a heme-containing (Hb-like) protein found in plants. It will be readily recognized by the research community without a prolonged explanation of the origin of the term, as is the case for ‘nonsymbiotic hemoglobin’. The classification system that has been established can essentially remain unchanged substituting Phytogb in place of nsHb. A guide to the new nomenclature, with reference to the existing terminology, is given in Table 1. A more detailed phylogenetic scheme, placing the known Phytogbs in the new nomenclature, is shown in Figure 1. Also, we propose that acronym for the species-specific Phytogbs corresponds to the first three binomial (i.e. genus and species) letters followed by the Phytogb type and phytogb number of copy. For example, the acronym for rice (Oryza sativa) Phytogb1.1 (see Table 1) corresponds to OrysatPhytogb1.1.

Figure 1. Phylogenetic representation of the novel nomenclature for land plant Phytogbs.

Note that Parasponia, Casuarina, Alnus and Myrica SymPhytogbs are intermediate between SymPhytogbs and Phytogbs1 and Phytogbs2 (see Table 1 for explanation). Figure modified from Garrocho-Villegas et al.²³ (reprinted with permission).