[
IUBMB Life,
2000]
In the past decade, important advances have been made in our knowledge of the composition of human RNase MRP and RNase P complexes. Both ribonucleoprotein particles function as endonucleases and contain RNA components that are structurally related. RNase MRP has been suggested to be involved in the processing of precursor rRNA; RNase P, in the maturation of tRNA. Here we give an overview of current data on the structure and function of human RNase MRP and RNase P particles, with emphasis on their molecular composition. At present, seven protein subunits, probably all associated with both ribonucleoprotein particles, have been isolated and their corresponding cDNAs cloned. Although no known structural motifs can be identified in the amino acid sequences of these proteins, the majority is clearly rich in basic residues. For two protein subunits, a cluster of basic amino acids have been shown to be involved in nucleolar accumulation, whereas another protein, which lacks such a region, probably enters the nucleolus by way of a piggyback mechanism. The binding regions for several of the protein subunits on the RNA have been identified, and the data have been used to create a putative structural model for the RNase MRP particle. The rather obscure situation concerning the association of the autoantigenic Th-40 protein and its possible relationship with one of the subunits, Rpp38, is discussed.
[
Ann N Y Acad Sci,
1992]
The freeze-fracture technique offers a unique view of intramembrane particles (IMPs), which derive from large membrane-associated molecules such as gap junctions, receptors, and ion channels. We are particularly interested in the gap junction (gj) and its role in intercellular communication. The anatomy of the soil nematode, C. elegans, has been studied extensively in serial thin sections and gjs have been noted in many cell types. Although gjs vary in frequency and extent, their appearance in sectioned material is rather uniform. The freeze-fracture technique can be used to identify and differentiate gjs according to IMP size, packing density, and preferred fracture face. For instance, in the planarian, Dugesia, this technique revealed three classes of gjs occurring in different tissues. The nematode usually fractures lengthwise; the fracture plane preferentially travels along membranes, splitting the unit membrane into two opposing halves (the P- and E-faces). Many tissues are recognizable: hypodermis, muscle, neurons, nerve cords, intestine, and so forth...