The kinetoplast—mitochondrion complex
The kinetoplast is situated close to the nucleus, and its shape and structural organization vary according to the developmental stage of the protozoan. The trypanoso- matids possess a unique and highly ramified mitochondrion. The kinetoplast appears as a dense structure and is made of kinetoplast DNA (kDNA), found in a specialized portion of the mitochondrion (Fig. 18.4) within the mitochondrial matrix, perpendicular to the axis of the flagellum.
Two types of circular DNA are present in the kinetoplast: minicircles and maxicircles. There are several thousand minicircles, which range in size from about 0.5 to 2.5 kb (depending on the species), and a few dozen maxicircles, which range from 20 to 40 kb.9 Together, maxicircles and minicircles represent approximately 30% of the total cellular genome.10 The minicircles present high heterogeneity but also have a conserved region where replication origin sites are localized. Minicircles also encode guide RNAs that modify the maxicircle transcripts by
Figure 18.4 Different views of the kinetoplast and the general organization of the trypomastigote (A), amastigote (B), and epimastigote (C) forms of Trypanosoma cruzi.
F: flagellum; K: kinetoplast; N: nucleus. The white arrows point to the area of adhesion of the flagellum to the cell body. White arrowheads point to profiles of the endoplasmic reticulum. Bar, 1 pm.
Source: After (A and B) De Souza, W. Growth and transformation of Trypanosoma cruzi.
In: Briggs AP, Coburn, JA, editors, Handbook of cell proliferation. Nova Science Publishers; 2009; (C) Rocha GM, Brandao BA, Mortara RA, Attias M, De Souza W, Carvalho TM. The flagellar attachment zone of Trypanosoma cruzi epimastigote forms. J Struct Biol 2006;154:89-99.
extensive uridylate insertion or deletion, a process known as RNA editing. The maxicircles are structurally and functionally analogous to the mitochondrial DNA. In situ analysis of the kinetoplast DNA structure showed that the network is formed not by circles, but by irregular polygonal structures (Fig. 18.5). The association of kDNA and mitochondrion membrane are made by filaments known as unilateral filaments. The connection between outer portion of the mitochondrial membrane to the basal body are made by another filaments. Together, the probasal body and the two cytoskeletal structures have been designated as the Tripartite Attachment Complex (TAC) of the kinetoplast.11-13
Figure 18.5 Organization of the kinetoplast DNA fibers in maxicircles and minicircles. The kinetoplast DNA network was dispersed in water, collected in a grid and shadowed at low angle with platinum. Bar, 0.3 pm.
Source: Courtesy of David Ptsrez-Morga.