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The Murine Homeo Domain Protein Hox 1.1

Complex regulatory processes in an animal cell are directed by proteins, which can for example act via mechanisms involving binding to one or more ligands and catalysis of a subsequent reaction. The appearance of only one or a few such proteins may be enough to redirect the program of the cell and induce a new phenotype, e.g. the change from a normal to a neoplastic type. Ideal targets for such proteins are sites on genomic DNA, which may be used to activate transcription, so that eventually a network of new interactions evolves. The class of DNA binding proteins, as for example transcription factors' and hormone receptors,2 has recently been investigated in great detail. Other proteins were studied intensively, before it became clear that they belong to the DNA binding proteins. Among the latter are the homeo domain proteins, encoded for by the homeo box containing genes.' These genes were originally discovered in the Drosophila genome as parts of genes involved in the organisation of the body plan, such as homeotic genes, segmentation genes and maternal effect genes.' The availability of molecular probes then led to the identification and isolation of homeo box containing genes in many different animals, including mice and humans.' In the murine genome probably around twenty homeo box containing genes are present. Two chromosomes, 6 and 11, carry clusters which have been termed Hox 1 and Hox 2, respectively.a In order to approach the function of these genes, their products have to be characterised. This paper summarises our information on the murine Hox 1.1 protein, the gene product of the first gene of the Hox 1 locus on chromosome 6 (REF. 9). We have isolated cDNA clones which allowed us to deduce the primary structure of the Hox 1.1 protein (FIGURE l)." In the following we will describe some aspects of the architecture of the protein as can be delineated from the amino acid sequence. Hox 1.1 consists of 229 amino acids, which add up to a relative molecular mass of 25,000. In the amino acid composition three regions of unusual amino acid composition are clearly evident, which probably are responsible for the main characteristics of the protein (FIGURE 2). The N-terminus of the Hox 1.1 protein (region a , FIGURE 2) contains 10% proline residues, an amino acid with profound influence on the secondary structure (see below). This region is followed by the homeo domain (region c), characterised by a content of 30% of lysines and arginines which supply the protein with a high number of positive charges. The third dominant region at the carboxyterminus (region e in FIGURE 2) adds a comparable amount of negative charges, being composed of a homogeneous stretch of 15 glutamic acids. The consequences of these primary structures for the secondary structure are approachable by computer