- Email: [email protected]
Membrane transport in the malaria-infected erythrocyte
Abstracts / Comparative Biochemistry and Physiology, Part B 126 (2000) S1-S108
$65
MEMBRANE T R A N S P O R T IN T H E MALARIA-INFECTED ERYTHROCYTE R.E. Martin 1, K.J. Saliba l, H.M. Staines 2, IC Kirk 1 IDivision o f Biocbemistry and Molecular Biology, Faculty o f Science, ANU, Canberra 0200, Australia and 2University Laboratory o f Physiology, Parks Road, Oxford OX1 3PT, U K The malaria parasite is a unicellular protozoan which, during the course of its complex lifeeycle, invades the erythroeytes of its host, resulting in the somewhat unusual situation of one ~ o t i c cell living inside another. The intracellular parasite grows and replicates at an enormous rate, and its requirement for metabolic and biosynthetic substrates far outstrips that of the normal ~fthroeyte. As it grows it ingests the host cell cytosol and digests the component proteins (predominantly haemoglobin). For some substrates required by the parasite (e.g. glucose), entry into the host cell via endogenous mmsporters in the erythrocyte membrane is sufficiently fast to meet the parasite's demands. However, for other essential nutrients required by the parasite, the host cell membrane either lacks endogenous transporters (as is the case for the essential vitamin pantothenate and the amino acid glutamate) or transports them too slowly to meet the parasite's requirements (as is the case for isolencine, the one amino acid that is absent fi'om haemoglobin and which the parasite is therefore unable to obtain from this source). For these solutes the entry into the host cell is wholly, or in part, via new permeation pathways induced by the parasite in the host cell plasma membrane. These pathways have the characteristics of anion-selective pores or channels which accommodate a wide range of solutes, up to 11-12 angstroms in diameter. They have a general preference for anions and eleetroneutral solutes but have, nevertheless, a significant permeability to both inorganic and organic monovalent cations, and their induction causes a dissipation of the normal Na+/K+ gradients across the erythroeyte membrane. Having gained entry into the host erythrocyte, nutrients are taken up by the inWacellular parasite via substrate-specific transporters in the parasite plasma membrane. The essential vitamin pantothenate enters the intracellular parasite via a low affinity H+:pantothenate symporter. Isoleucine is taken up by the parasite via a high affinity transporter, predominantl~yin exchange for leucine, the most abundant amino acid in haemoglobin.
THE
EVOLUTION
OF
AVIAN
HETEROTHERMY:
NEW
INSIGHTS
FROM
THE
MOUSEBIRDS (COLIIFORMES). Andrew McKechnie and Barry Lovegrove School o f Botany and Zoology, University o f Natal, Private Bag X01, Scottsville 3209, South Africa Patterns of normothermy and heterothermy in two mousebird species differed in several respects fi'om typical endothermic patterns. Primarily, heterothermic responses in speckled mousebirds (Colius striatus) to restricted food lacked the entry and maintenance phases characteristic of typical torpor bouts. Furthermore, there was no evidence of the defence of a torpor body temperature setpoint. The body temperature of the birds typically decreased throughout the restphase, readfing a minimum shortly before the commencement of arousal. The lowest body temperature recorded in a C striatus which was able to spontaneously arouse was 18.20C. Similar patterns were observed in single white-backed mousebirds (C. colius) exposed to low ambient temperatures. These observations suggest that mousebirds may exhibit "proto-torpor", a form of torpor intermediate between hypothesized ancestral wide-amplitude Tb cycling and modern heterothermy. Clustering behaviour was necessary for the defence of a constant body temperature during the rest-phase in both species, and reduced the extent ofheterothermic responses to reslricted food in C. striatus. In C. colius, clustering behavior was also necessary for the avoidance of pathological hypothermia at low ambient temperatures. These observations suggest that clustering behavior constitutes an obligatory component of thermoregulation in mousebirds. The evolution of typical avian torpor appears to have been arrested in the Coliiformes by the development of sociality and clustering.
$65
MEMBRANE T R A N S P O R T IN T H E MALARIA-INFECTED ERYTHROCYTE R.E. Martin 1, K.J. Saliba l, H.M. Staines 2, IC Kirk 1 IDivision o f Biocbemistry and Molecular Biology, Faculty o f Science, ANU, Canberra 0200, Australia and 2University Laboratory o f Physiology, Parks Road, Oxford OX1 3PT, U K The malaria parasite is a unicellular protozoan which, during the course of its complex lifeeycle, invades the erythroeytes of its host, resulting in the somewhat unusual situation of one ~ o t i c cell living inside another. The intracellular parasite grows and replicates at an enormous rate, and its requirement for metabolic and biosynthetic substrates far outstrips that of the normal ~fthroeyte. As it grows it ingests the host cell cytosol and digests the component proteins (predominantly haemoglobin). For some substrates required by the parasite (e.g. glucose), entry into the host cell via endogenous mmsporters in the erythrocyte membrane is sufficiently fast to meet the parasite's demands. However, for other essential nutrients required by the parasite, the host cell membrane either lacks endogenous transporters (as is the case for the essential vitamin pantothenate and the amino acid glutamate) or transports them too slowly to meet the parasite's requirements (as is the case for isolencine, the one amino acid that is absent fi'om haemoglobin and which the parasite is therefore unable to obtain from this source). For these solutes the entry into the host cell is wholly, or in part, via new permeation pathways induced by the parasite in the host cell plasma membrane. These pathways have the characteristics of anion-selective pores or channels which accommodate a wide range of solutes, up to 11-12 angstroms in diameter. They have a general preference for anions and eleetroneutral solutes but have, nevertheless, a significant permeability to both inorganic and organic monovalent cations, and their induction causes a dissipation of the normal Na+/K+ gradients across the erythroeyte membrane. Having gained entry into the host erythrocyte, nutrients are taken up by the inWacellular parasite via substrate-specific transporters in the parasite plasma membrane. The essential vitamin pantothenate enters the intracellular parasite via a low affinity H+:pantothenate symporter. Isoleucine is taken up by the parasite via a high affinity transporter, predominantl~yin exchange for leucine, the most abundant amino acid in haemoglobin.
THE
EVOLUTION
OF
AVIAN
HETEROTHERMY:
NEW
INSIGHTS
FROM
THE
MOUSEBIRDS (COLIIFORMES). Andrew McKechnie and Barry Lovegrove School o f Botany and Zoology, University o f Natal, Private Bag X01, Scottsville 3209, South Africa Patterns of normothermy and heterothermy in two mousebird species differed in several respects fi'om typical endothermic patterns. Primarily, heterothermic responses in speckled mousebirds (Colius striatus) to restricted food lacked the entry and maintenance phases characteristic of typical torpor bouts. Furthermore, there was no evidence of the defence of a torpor body temperature setpoint. The body temperature of the birds typically decreased throughout the restphase, readfing a minimum shortly before the commencement of arousal. The lowest body temperature recorded in a C striatus which was able to spontaneously arouse was 18.20C. Similar patterns were observed in single white-backed mousebirds (C. colius) exposed to low ambient temperatures. These observations suggest that mousebirds may exhibit "proto-torpor", a form of torpor intermediate between hypothesized ancestral wide-amplitude Tb cycling and modern heterothermy. Clustering behaviour was necessary for the defence of a constant body temperature during the rest-phase in both species, and reduced the extent ofheterothermic responses to reslricted food in C. striatus. In C. colius, clustering behavior was also necessary for the avoidance of pathological hypothermia at low ambient temperatures. These observations suggest that clustering behavior constitutes an obligatory component of thermoregulation in mousebirds. The evolution of typical avian torpor appears to have been arrested in the Coliiformes by the development of sociality and clustering.