The nuclear envelope allow the nucleus to control its contents , and separate them from the rest of the cytoplasm where necessary . This is important for controlling processes on either side of the nuclear membrane . In most cases where a cytoplasmic process need to be restricted , a key participant is removed to the nucleus , where it interact with transcription factors to downregulate the production of certain enzymes in the pathway . This regulatory mechanism occur in the case of glycolysis , a cellular pathway for breaking down glucose to produce energy . Hexokinase is an enzyme responsible for the first the step of glycolysis , forming glucose @-@ 6 @-@ phosphate from glucose . At high concentrations of fructose @-@ 6 @-@ phosphate , a molecule made later from glucose @-@ 6 @-@ phosphate , a regulator protein removes hexokinase to the nucleus , where it form a transcriptional repressor complex with nuclear proteins to reduce the expression of genes involved in glycolysis .