The role and expression of β-glucosidase during spore germination of mutant and wild-type Dictyostelium discoideum

Abstract

Dormant spores of Dictyostelium discoideum are contained in a viscous matrix of extracellular material at the top of sorocarps. Analysis of crude extracts of the sorus demonstrates that there is a higher specific activity of β-glucosidase in the matrix than in the dormant spores. Polyacrylamide gel electrophoresis of the matrix material reveals the presence of β-glucosidase 2 with trace amounts of β-glucosidase 1. In the dormant spores only β-glucosidase 2 is detected. Upon heat activation of the spores, β-glucosidase 2 activity decreases to approximately 25% of the activity found in the preheated spores. The enzyme appears to have a surface location in dormant spores, since trypsin treatment will destroy all of the β-glucosidase 2 activity. The normal germination of dormant spores after trypsin treatment suggests that β-glucosidase 2 activity is not required for activation, swelling, or emergence processes.During the emergence of myxamoebae, β-glucosidase 2 is replaced by a newly synthesized enzyme, β-glucosidase 1. The formation of this new enzyme requires both RNA and protein syntheses. The complete germination of a β-glucosidase 1 minus mutant, however, casts doubt on the necessity for β-glucosidase1 activity during emergence. This is further supported by the normal germination of strain SG1 which contains four- to five-fold higher β-glucosidase activity than that found in wild-type amoebae at the end of the germination period. In addition, strain SG1 may be activated with a number of techniques which results in a separation of the time of β-glucosidase synthesis from the time of myxamoebae emergence. Furthermore, spores (which have been blocked from emerging by cycloheximide treatment) will slowly release spheroplasts when treated with only pronase and D. discoideum cellulase. However, addition of D. discoideum β-glucosidase 1 to the cellulose–pronase-treated spores does accelerate the rate of spheroplast formation. Thus, β-glucosidase 1 activity may only play a minor and dispensable role in D. discoideum spore germination.