Larger, insoluble materials like proteins are often packaged by the various organelles that are involved in their production in vesicles. These vesicles are too large to travel around the cell by diffusion so are actually carried/dragged along the microtubules by motor proteins (Eg Kinesin). These motor proteins literally walk along the microtubules dragging the vesicles behind them. The direction in which they walk is determined by the polarity of the microtubule heterodimer.
